OBD-II Code P1940: Brake Assist / Pressure Sensor Fault

What Does P1940 Mean?

P1940 is a manufacturer-specific code signaling a problem with the brake booster pressure sensor. Your car's computer (PCM) detects the sensor signal is incorrect, missing, or outside its expected range. This critical safety component informs the computer about available vacuum for the power brake assist system, directly influencing electronic stability control and ABS.

Technical definition: The official definition of P1940 varies by manufacturer. For Ford and BMW, it indicates a 'Brake Pressure Sensor A Circuit' fault or 'Brake Assist Fault'. For Mercedes-Benz and Volkswagen, it points to a 'Glow Plug Control Module' or 'Battery Control Module' fault. This guide focuses on the common brake-related definition while detailing important manufacturer-specific variations.

Can I Drive With P1940?

Yes, But With Caution. Your mechanical brakes still work, but stopping requires significantly more pedal effort. Without power assist, your stopping distance doubles or triples, turning a routine stop into a potential collision. Drive with extreme caution and only for a short distance directly to a repair shop.

Common Causes

• Failed Brake Booster Pressure Sensor (Very Common) — The sensor itself is the most frequent point of failure. It wears out internally, suffers from contamination (especially brake fluid on BMWs), or fails electronically, sending incorrect voltage signals to the PCM.
• Vacuum Leak (Common) — The brake booster requires engine vacuum to function. A cracked, disconnected, or deteriorated vacuum hose between the engine and the booster causes a loss of power assist and triggers this code.
• Failed Mechanical/Electric Vacuum Pump (Common) — On engines that don't produce enough natural vacuum (Ford EcoBoost, BMW Valvetronic), a separate pump supplies the brake booster. If this pump fails or its seals leak, it won't generate enough vacuum, leading to a hard pedal.
• Damaged Wiring or Connectors (Common) — Wires leading to the brake pressure sensor fray, corrode, short, or break. The connector itself loosens, becomes contaminated with dirt or oil, or has its pins back out, disrupting the signal.
• Defective Brake Booster Check Valve (Less Common) — A small one-way valve at the booster holds vacuum. If this valve fails, vacuum is lost when the engine is off or under low vacuum conditions, making the pedal hard to press.
• Failing Brake Booster (Less Common) — The internal diaphragm of the brake booster ruptures. This causes a major vacuum leak, accompanied by a hissing sound when the brake is pressed, and a complete loss of power assist.
• Low or Contaminated Brake Fluid (Rare) — Extremely low or contaminated brake fluid causes hydraulic issues. On some models, spilled fluid during service directly contaminates and destroys the pressure sensor located below the reservoir.
• Faulty Control Module (PCM/ABS) (Rare) — The problem lies with the Powertrain Control Module (PCM) or ABS module that reads the signal. This is a last-resort diagnosis considered only after ruling out all other possibilities.

Symptoms

• Hard Brake Pedal — The brake pedal is very difficult to push down, requiring much more leg strength to stop the car.
• Check Engine Light or Warning Messages — The Check Engine Light illuminates. You also see warning lights for the ABS, Traction Control (DTC), or a specific 'Brake Assist Fault' message on the dashboard.
• Increased Stopping Distance — Without power assist, the distance it takes to bring the vehicle to a complete stop increases dramatically, creating a serious safety risk.
• Hissing Noise from Pedal Area — A failing brake booster diaphragm or a leak in the main vacuum hose creates an audible hissing sound when you press or release the brake pedal.
• Erratic or High Idle Speed (also visible on scanner) — A significant vacuum leak in the brake booster or its supply hose allows unmetered air into the engine, disrupting the air-fuel mixture and causing a rough, high, or fluctuating idle.

Diagnostic Flowchart

Tap your situation to follow the diagnostic path that matches what you're seeing on this code.

Common Fixes & Costs

• Replace Brake Booster Pressure Sensor — Parts: $45-$110, Labor: $85-$150, ~0.8 hr book time (Intermediate)
• Replace Cracked or Leaking Vacuum Hose — Parts: $20-$60, Labor: $50-$120, ~0.5 hr book time (DIY)
• Replace Mechanical Vacuum Pump (Ford/BMW) — Parts: $100-$300, Labor: $125-$250, ~1.5 hr book time (Intermediate)
• Replace Brake Booster — Parts: $150-$650, Labor: $175-$450, ~2.5 hr book time (Professional)
• Replace Glow Plug Control Module (Mercedes-Benz) — Parts: $160-$250, Labor: $75-$110, ~0.8 hr book time (Intermediate)
• Repair Wiring or Connector — Parts: $5-$25, Labor: $100-$300, ~2.0 hr book time (Professional)

DIY vs Professional

• Replace Brake Booster Pressure Sensor — Beginner: Yes, if easily accessible.
  Tools: Basic hand tools (socket set, pliers), flashlight.
• Replace Cracked or Leaking Vacuum Hose — Beginner: Yes.
  Tools: Pliers, hose pick/remover.
• Replace Mechanical Vacuum Pump — Beginner: No.
  Tools: Extensive socket set, torque wrench, gasket scrapers.
• Replace Brake Booster — Beginner: No.
  Tools: Extensive socket set, wrenches, torque wrench, brake fluid, brake bleeding equipment.
• Replace Glow Plug Control Module (Mercedes-Benz) — Beginner: Yes, it's often accessible.
  Tools: Basic socket set.
• Repair Wiring or Connector — Beginner: No.
  Tools: Multimeter, wire strippers, crimpers, soldering iron, heat shrink, depinning tools.

Used vs. New Parts: Buying Guide

When a used part is worth it: For purely mechanical parts like vacuum hoses or a complete brake booster from a low-mileage vehicle, a used part is cost-effective. For electronic components, the risk of premature failure is higher.

Donor-vehicle mileage cap: roughly under 60000 miles for the part to have meaningful remaining life.

Donor quality checklist:

• Verify the part number matches exactly.
• For vacuum pumps, avoid parts showing oil seepage.
• For electronic sensors, check for corrosion on pins or cracks in the housing.
• Prefer donors from dry, salt-free climates.

Decision logic:

• If The part is a simple vacuum hose or check valve → Used is acceptable if in good physical condition.
• If The part is an electronic sensor, vacuum pump, or glow plug module → New (OEM or reputable aftermarket) is strongly recommended due to high failure rates of used electronics.
• If The part is a complete brake booster assembly → Used from a low-mileage donor is a viable option, but carries the risk of a worn internal diaphragm.

Warranty tradeoff: Used parts from a salvage yard typically have a 30-90 day warranty covering only the part. New aftermarket parts offer a 1-year to limited lifetime warranty. New OEM parts carry a 1-2 year warranty.

Worst-case if a used part fails: $200-$500. This represents the cost of repeat labor to diagnose and replace a faulty used part.

What Happens If You Wait — Timeline

1. Immediate: Code P1940 is set. Check Engine Light and/or ABS/Brake Assist warnings illuminate. Brake pedal becomes hard, requiring significantly more effort to stop. Vehicle enters 'limp mode'. (MPG impact: 0-5%% · Added cost: $0)
2. 0-1 month: Extreme safety risk due to increased stopping distance, which is 2-3 times longer than normal. If caused by a vacuum leak, a rough or high idle develops, and fuel economy drops. (MPG impact: 5-10%% · Added cost: $25-$100 in wasted fuel. Cost of an at-fault collision could be thousands.)
3. 1-4 months: If caused by a persistent vacuum leak, the engine runs lean, causing hesitation and stress on ignition components. If caused by a leaking mechanical vacuum pump, the oil leak worsens, starving the pump of lubrication. (MPG impact: 10-15%% · Added cost: $100-$300 in wasted fuel. Potential for secondary damage to spark plugs or coils.)
4. 4+ months: Catastrophic failure potential. A long-term lean condition damages the catalytic converter ($1200+). A seized mechanical vacuum pump causes internal engine damage to the camshaft ($1500+). Ignoring limp mode leads to transmission damage. (MPG impact: 15-25%% · Added cost: $1200-$3000+)

Cost of Not Fixing It

• Immediate: Extreme safety risk. Stopping distance increases 2-3x, making a collision much more likely. The vehicle enters 'limp mode,' severely reducing engine power. (Added cost: Cost of an at-fault accident (deductible, insurance increase, injury).)
• 0-3 months: If caused by a vacuum leak, the engine runs lean, causing poor fuel economy, hesitation, and rough idle. This causes added stress on the ignition system. (Added cost: $50-$200 in wasted fuel, plus potential for spark plug/coil failure.)
• 3+ months: A persistent lean condition damages the catalytic converter. On Ford/BMW models, a failing vacuum pump seizes, causing damage to the engine or camshaft. (Added cost: $1200-$2800 for a new catalytic converter; $1500+ for engine damage from a seized pump.)

Diagnosis Steps

1. Scan for All Codes
   Use an OBD-II scanner to confirm P1940 and check for codes in all vehicle modules (ABS, BCM). Note the exact code definition provided. The presence of other codes is a critical clue to the root cause.
   Tools: OBD-II Scanner (Beginner)
2. Test Brake Booster Vacuum Retention
   With the engine off, pump the brake pedal 4-5 times to deplete stored vacuum. The pedal becomes firm. Hold the pedal down with steady pressure and start the engine. The pedal drops slightly (about an inch) as vacuum builds. If it doesn't move, the booster is not getting or holding vacuum.
   Tools: None (Beginner)
3. Visual Inspection
   Inspect the brake booster, connected vacuum hoses, and wiring going to the pressure sensor. Look for cracks in hoses, loose fittings, and frayed wires. On Ford EcoBoost engines, inspect the auxiliary vacuum pump for oil leaks, a sign of seal failure.
   Tools: Flashlight (Beginner)
4. Check the Brake Booster Check Valve
   Remove the check valve from the booster. Blow air through it from both directions. Air only passes in one direction (away from the booster, toward the engine). If you blow through it both ways, or not at all, replace the valve.
   Tools: Pliers (Intermediate)
5. PRO TIP: Test Vacuum Supply
   Disconnect the vacuum hose at the brake booster and connect a vacuum gauge. Start the engine and let it idle. A healthy engine/vacuum pump produces a steady vacuum of 16-21 in. Hg. If the reading is low or erratic, the problem lies with the vacuum source, not the booster or sensor.
   Tools: Vacuum Gauge (Advanced)
6. PRO TIP: Analyze Live Scan Tool Data
   Connect a professional scan tool and monitor the PID for 'Brake Booster Pressure'. With Key On, Engine Off, the reading is near 0 in. Hg. When the engine starts, the value quickly changes to show a deep vacuum (16-21 in. Hg at idle). If the value does not change, it confirms a fault in the sensor, circuit, or vacuum supply.
   Tools: Professional OBD-II Scan Tool (Advanced)
7. PRO TIP: Test Sensor Circuit Voltage
   Identify the sensor's three wires: 5V reference, ground, and signal. With key on, engine off (KOEO), confirm a steady 5V reference and good ground (<0.1V). Backprobe the signal wire. KOEO voltage is high (~4.5V). When the engine starts, signal voltage drops to 1.0-1.5V at idle. A signal stuck high indicates a bad sensor or lack of vacuum. A signal stuck low indicates a failed sensor or wiring issue.
   Tools: Multimeter, Backprobe Kit (Advanced)
8. Check Wiring for Continuity and Shorts
   Disconnect the sensor and control module connectors. Use a multimeter to check for continuity on the power, ground, and signal wires between the two connectors (<1 ohm is ideal). Check for shorts by testing for continuity between each wire and a chassis ground, and between the wires themselves (should read OL).
   Tools: Multimeter (Advanced)
9. Perform a Smoke Test
   If a vacuum leak is suspected but cannot be found visually, a smoke machine forces low-pressure smoke into the intake and vacuum system. Leaks are revealed by visible smoke seeping from cracked hoses, bad gaskets, or a ruptured booster diaphragm.
   Tools: Smoke Machine (Professional)

When This Code Triggers (Freeze-Frame Conditions)

• Engine Coolant Temp: 170-200°F (Fully warmed up. The code often sets after the engine reaches normal operating temperature.)
• Engine RPM: 700-1200 RPM (At or near idle, often during stop-and-go traffic. A large vacuum leak from the booster causes RPMs to fluctuate wildly when the brake is pressed.)
• Brake Pedal Status: Applied (The fault is detected when the brake pedal is pressed, and the sensor's reading does not correlate with the expected change in vacuum.)
• Vehicle Speed: 0-25 mph (Low-speed driving or stationary. The demand on the booster and subsequent vacuum change is most noticeable to the PCM under these conditions.)

Related Codes

• C1288 — This Ford-specific code for 'Pressure Transducer Main/Primary Circuit Failure' points directly to an electrical fault with the sensor itself. If you have both C1288 and P1940, the sensor or its wiring is the problem.
• C101A — A Ford code for 'Brake Booster Pressure Sensor Circuit'. This code confirms the fault is within the sensor's circuit, narrowing the diagnosis.
• P050F — Stands for 'Brake Assist Vacuum Too Low'. This code confirms the problem is a lack of vacuum, not the sensor's electrical circuit. Prioritize finding a vacuum leak or inspecting the vacuum pump.
• P0670 — On Mercedes-Benz diesels, P1940 is frequently paired with P0670 ('Glow Plug Control Module Circuit Fault'). P0670 causes hard starting and white smoke. If these symptoms are present without brake issues, the problem is the glow plug system.

Climate & Environmental Factors

• Cold Weather: In freezing temperatures, moisture collected in the brake booster check valve or vacuum lines freezes, causing a blockage and a hard pedal on startup. Rubber diaphragms become stiff, leading to temporary vacuum leaks.
• High Altitude: Engines produce less vacuum at higher altitudes. A marginal problem like a small leak or a weak vacuum pump becomes much more noticeable at altitude, triggering the P1940 code where it might not at sea level.
• High Humidity: Humid environments increase the likelihood of moisture collecting inside the vacuum system, contributing to cold weather freezing issues.

How to Talk to a Mechanic About This Code

Say this: "I have a P1940 code and a hard brake pedal. I'd like to schedule a diagnostic. Please start by testing the vacuum supply at the booster and checking the sensor's live data before recommending a part replacement."

This signals you've done research and prevents the shop from replacing the sensor without confirming if the problem is a cheaper vacuum leak or a more expensive vacuum pump.

Avoid saying:

• 'My brakes feel weird, can you check them?'
• 'My check engine light is on, just fix it.'
• 'I think I need a new brake booster.'

Questions to ask before authorizing the repair:

• What was the vacuum reading in inches of mercury (in. Hg) at the booster hose?
• Did the sensor's voltage or pressure reading on the scan tool change when the engine was started?
• If you're recommending a vacuum pump replacement, is the old one leaking oil?
• Can you show me the cracked hose or the failed part?

Where to Take It: Dealer vs Independent vs Chain

• Dealer: Recommended for German brands where the code has a unique meaning or requires special programming. For others, an independent shop is more cost-effective.
  Best for: Vehicles under warranty., Manufacturer-specific definitions of P1940 (Mercedes glow plug module, VW battery module)., Complex electrical issues or repairs requiring vehicle-specific software.
  Downsides: Highest labor rates, often 1.5-2x more than independent shops., Recommends replacing an entire assembly when a smaller component has failed. (Typical cost: +50% vs. baseline)
• Independent Shop: Best overall fit for the majority of P1940 cases, offering a balance of expertise and value.
  Best for: Out-of-warranty vehicles with the common brake-related P1940 fault., Diagnosing and repairing vacuum leaks, sensor replacements, and vacuum pump swaps.
  Downsides: Quality varies greatly; look for ASE certifications and good reviews., Lacks specific software for German vehicle quirks. (Typical cost: +0% vs. baseline)
• Chain Shop: AVOID for initial diagnosis. The risk of misdiagnosis is very high. Only consider for a simple part swap if you are 100% certain of the failed component.
  Best for: Simple, clear-cut part replacements if the diagnosis is already certain.
  Downsides: Technician skill varies dramatically., High pressure to upsell unnecessary services., Ill-equipped for in-depth vacuum or electrical diagnostics. (Typical cost: -10% vs. baseline)

When to Walk Away From the Repair

If the estimated repair cost exceeds 40-50% of the car's private-party value, seriously consider selling or trading in the vehicle.

• Car worth $4000, fix is $1100: Borderline. The repair is ~28% of the car's value. Get a second opinion before authorizing, but it's likely worth fixing if the rest of the car is in good shape.
• Car worth $15000, fix is $450: Fix it. This is a routine repair cost for a vehicle of this value.
• Car worth $2500, fix is $1500: Walk away. The repair cost is 60% of the car's value. This is not economically viable.

What Scan Tool You Need for This Code

Minimum: A scanner that reads live data PIDs for the ABS and engine modules to monitor 'Brake Booster Pressure'.

A basic $20 code reader only shows 'P1940'. It cannot show the live sensor data essential to determine if the sensor is bad or if the problem is the vacuum supply.

Budget: BlueDriver Pro (~$100) — Connects to your phone to display live data, including brake booster pressure. It provides freeze-frame data and reads enhanced codes from the ABS system.

Mid-range: Foxwell NT510 Elite (~$180) — A handheld scanner providing manufacturer-specific diagnostics. It reads brand-specific codes and live data PIDs with greater accuracy than generic tools.

Professional: Autel MaxiCOM MK808 / MK808BT (~$450-550) — Offers full system diagnostics, live data graphing, and bidirectional controls to command components like an electric vacuum pump.

Rent vs buy: For a one-time diagnosis, use the free loaner tool program from auto parts stores. Buy a scanner only if you plan to perform diagnostics regularly.

How to Clear the Code After You Fix It

1. Use an OBD-II scan tool to clear all fault codes.
2. Disconnecting the battery is not recommended as it clears adaptive memory and resets all monitors to 'Not Ready'.
3. Perform a complete drive cycle to allow the onboard diagnostic systems to verify the repair.

Drive cycle (~25 minutes): 1. Cold start and idle for 2-3 minutes. 2. Drive in stop-and-go traffic for 10 minutes. 3. Accelerate to a steady 55-60 mph and hold for 5-10 minutes. 4. Safely decelerate to 20 mph without braking, then accelerate back to 55 mph. 5. Allow the vehicle to cool down completely and repeat if monitors are not set.

Readiness monitors affected: Comprehensive Component Monitor (CCM), Evaporative System (EVAP), Catalyst (CAT)

Before emissions retest: drive at least 50 miles to fully set monitors.

Watch out for:

• Clearing the code without performing a drive cycle results in a 'Not Ready' status at an emissions test.
• The code returns immediately if the underlying mechanical or electrical issue is not fully resolved.

Will This Fail Emissions / State Inspection?

Yes — this code typically fails an OBD-II emissions inspection.

• California: An active Check Engine Light for P1940 is an automatic failure. All readiness monitors must be 'Ready'. After repair, a full drive cycle is required before re-testing.
• New York: The NYS inspection includes an OBD-II scan. An illuminated Malfunction Indicator Lamp (MIL) due to P1940 results in an immediate test failure.
• Texas: An active P1940 code causes the vehicle to fail the OBD-II portion of the inspection. Simply clearing the code is not enough; readiness monitors must be set to 'Ready'.

Most Commonly Affected Vehicles

• Ford Focus, Escape, Edge, F-150 (2012-2018) — Very common. Caused by a failed pressure sensor, a leaking auxiliary vacuum pump on EcoBoost models, or a faulty canister purge valve (Recall 18S32).
• BMW 3-Series (E90), 5-Series, X3, X5 (2006-2013) — Triggers a 'Brake Assist Fault' warning. Almost always a failed brake booster pressure sensor, often caused by contamination from spilled brake fluid. BMW issued an extended warranty for the electric vacuum pump on some models.
• Mercedes-Benz Sprinter, E-Class (Diesel) (2007-2016) — P1940 means 'Glow Plug Control Module Fault'. Pre-mid-2012 models use 4.4V steel glow plugs, while later models use 7V ceramic plugs. Control modules are different and must not be interchanged.
• Volkswagen / Audi Jetta, Golf, A4 (2008-2015) — Indicates a brake booster sensor fault, but is also defined as 'Battery Control Module - No Communication'. This module has 'Component Protection' and requires dealer programming.
• Chevrolet / GMC Silverado, Sierra, Tahoe (2007-2014) — Sets this code when the brake booster vacuum sensor fails. Aftermarket and OEM sensors are widely available.
• Hyundai Elantra, Santa Fe (2020-2022) — P1940 is uncommon. Its presence strongly suggests a cascading failure from another system. Check for TSBs like 22-01-076H or 21-01-048H.
• Kia Forte, Sorento (2019-2023) — P1940 is not a primary code. Owners report various issues (wheel speed sensor, MAF sensor) causing limp mode and multiple warnings. Diagnose other system codes first.
• Nissan Altima, Pathfinder, Rogue (2013-2018) — Triggered by a faulty brake booster pressure sensor. The repair is straightforward sensor replacement.

Manufacturer-Specific Notes

• Mercedes-Benz: For diesel engines, P1940 means 'Glow Plug Control Module Fault'. Symptoms are hard starting and white smoke, not a hard brake pedal. Pre-mid-2012 engines use different voltage glow plugs and modules than later models; they are not interchangeable.
• Volkswagen / Audi: P1940 means 'Battery Control Module - No Communication'. This is an anti-theft protected module. A replacement requires online programming by a dealer or specialist to function.
• BMW: Logs this as 'Brake Assist Fault'. It is almost always a failed pressure sensor located under the brake fluid reservoir, prone to failure from fluid contamination. BMW also issued an extended warranty for the electric vacuum pump on certain models.
• Ford: On EcoBoost models, a dedicated mechanical vacuum pump is a known failure point, leaking oil before failing. Be aware of recall 18S32 for 2012-18 Focus models where a stuck canister purge valve creates excessive vacuum.
• Hyundai / Kia: P1940 is a 'ghost code' triggered by an unrelated system failure. Check for active TSBs related to software updates or other sensor failures before attempting brake system repairs.

Real Owner Stories

2016 Ford F-150 5.0L with P1940 & P1289

Truck went into limp mode with an 'Engine Coolant Over Temperature' error on the dash, despite the coolant level being normal. The brake pedal was not noticeably hard.

What they tried:

1. Towed to dealer.

Outcome: Dealer diagnosed and replaced a faulty Cylinder Head Temperature (CHT) sensor. The part was inexpensive (around $45) but not commonly stocked. After replacement and clearing the codes, the issue was resolved.

Lesson: On some Fords, a CHT sensor failure causes bizarre cascading codes and limp mode that seem unrelated, like P1940. If you get an overheating warning without actual signs of overheating, suspect the CHT sensor first.

2008 BMW E90 328i with 'Brake Assist Fault'

Brake assist fault warning appeared on the iDrive screen, accompanied by a very hard brake pedal, especially noticeable at low speeds.

What they tried:

1. Owner initially suspected the brake booster itself or the vacuum pump.
2. A visual inspection under the hood revealed nothing obvious.

Outcome: A BMW-specific forum search pointed towards the brake booster pressure sensor located under the brake fluid reservoir. Brake fluid had leaked onto the sensor's connector during a past service, causing corrosion. Replacing the sensor (~$90 part) and cleaning the connector resolved the fault.

Lesson: On BMWs, the location of the pressure sensor makes it highly vulnerable to contamination from spilled brake fluid. Inspect this sensor for fluid damage before assuming a more expensive pump or booster failure.

2008 Mercedes Sprinter (Diesel) with P1940

Check Engine Light on, but the brakes worked perfectly fine. The van was becoming difficult to start in the mornings and would run rough with some white smoke for the first minute.

What they tried:

1. Initial code scan showed P1940, which generic online searches linked to brake problems.
2. Owner replaced all glow plugs, but the code and symptoms returned shortly after.

Outcome: Further research revealed that P1940 on this vehicle means 'Glow Plug Control Module Fault'. The module, located under the battery, was the actual culprit. Replacing the module (a ~$150 part) fixed the hard starting and cleared the code.

Lesson: Always verify the code definition for your specific make and engine. On many European diesels, P1940 has nothing to do with brakes. The symptoms (hard starting, rough cold idle) are the key clue.

How to Prevent This Code From Triggering

• Periodically inspect brake vacuum hoses. (Every oil change) — Visually check for cracks, swelling, or soft spots in the vacuum hoses connecting the engine, vacuum pump, and brake booster. A simple visual check catches a deteriorating hose before it fails completely.
• Be careful during brake fluid service. (During brake fluid flushes) — On vehicles where the pressure sensor is located near the master cylinder, spilled brake fluid is a primary cause of sensor failure. Use a funnel and rags to prevent fluid from dripping onto surrounding wiring.
• Change engine oil on schedule with correct grade. (Per manufacturer schedule (e.g., 5,000-10,000 miles)) — On engines with a mechanically-driven vacuum pump, the pump is lubricated by engine oil. Using the correct oil and changing it on time prevents sludge buildup and wear that leads to premature pump failure.
• Address engine oil leaks promptly. (As needed) — Oil leaking from other components drips onto and degrades the rubber vacuum hoses and electrical connector seals for the brake system, leading to vacuum leaks or electrical faults.

Frequently Asked Questions

Is it safe to drive my car with code P1940?

It is extremely unsafe. The lack of power assist means your stopping distance doubles or triples. Drive only to a nearby repair shop with extreme caution.

What's the most common mistake when diagnosing P1940?

The biggest mistake is replacing the brake booster pressure sensor without testing the vacuum supply first. Use a vacuum gauge to confirm you have 16-21 in. Hg of vacuum at the booster. The second mistake is ignoring manufacturer-specific definitions, like diagnosing brakes on a diesel Mercedes.

My Ford EcoBoost has P1940 and a hard pedal, but no obvious vacuum leaks. What's next?

Inspect the mechanical vacuum pump mounted on the rear of the cylinder head. Look for signs of oil leakage on the pump body. An oil leak indicates a failing seal and imminent pump failure.

Where is the brake booster pressure sensor located?

It is often mounted directly on the brake booster itself or in-line on the vacuum hose running from the engine. On some BMWs, it is located directly under the brake fluid reservoir.

How much does it cost to fix P1940?

Replacing the pressure sensor typically costs $130-$260, while a failed vacuum pump on a Ford runs $225-$550. For a diesel Mercedes, a glow plug module replacement is about $250-$300. A full brake booster replacement is the most expensive, often running $325-$1,100+.

Can I replace the brake booster pressure sensor myself?

If the sensor is easily accessible on the booster or vacuum line, it is an intermediate-level DIY job. If it requires removing other components, stop DIY and tow to a shop. Always disconnect the battery before working on electrical sensors.

My car is a diesel Mercedes and I have P1940, but my brakes feel fine. Why?

On your vehicle, P1940 indicates a 'Glow Plug Control Module Fault' and does not relate to the brakes. Your symptoms are difficulty starting, a rough idle when cold, and white exhaust smoke.

What is 'limp mode' and can P1940 cause it?

Limp mode is a self-preservation function where the computer drastically reduces engine power to prevent damage. Because brake assist is a critical safety system, a P1940 fault triggers limp mode on many modern vehicles.

Key Takeaways

• P1940 triggers a hard brake pedal, most commonly caused by a failed brake booster pressure sensor or a vacuum leak.
• Verify your manufacturer's specific definition; P1940 means 'Glow Plug Module Fault' on a diesel Mercedes and 'Battery Control Module Fault' on a VW.
• Do not drive with a P1940 code; the lack of power assist doubles or triples your stopping distance, creating an immediate collision risk.
• Inspect the mechanical vacuum pump for oil leaks on Ford EcoBoost and BMW engines, as seal failure is a primary trigger for this code.
• Test the vacuum supply with a gauge before replacing any parts; a healthy system must pull 16 to 21 in. Hg of vacuum at the booster.