OBD-II Code P1738: Automatic Transmission Clutch Pressure Switch Malfunction

What Does P1738 Mean?

P1738 means your car's computer detected an electrical fault in the 2nd clutch pressure switch circuit. This switch monitors hydraulic fluid pressure for 2nd gear. When the computer receives an unexpected voltage signal from this switch, it triggers the check engine light.

Technical definition: For Honda and Acura, the official definition is 'A/T 2nd Clutch Transmission Fluid Pressure Switch Circuit Malfunction'. The Powertrain Control Module (PCM) triggers this when it detects an out-of-range voltage from the switch. On Mazda vehicles, it indicates 'Automatic Transaxle Internal Malfunction', pointing to sticking internal oil pressure switches inside the valve body.

Can I Drive With P1738?

Yes, But With Caution. Yes, but only for short distances. The transmission often shifts harshly, hesitates, or locks into a single gear ('limp mode'), limiting speeds to 30-40 mph. Driving more than 50 miles risks severe internal transmission damage and overheating. Limp mode also forces high engine RPMs, which damages the catalytic converter over time—turning a $150 sensor replacement into a $2,500 repair.

Common Causes

• Failed 2nd Clutch Pressure Switch (Honda/Acura) (Very Common) — The external switch fails electrically, sticks, or leaks transmission fluid, sending an incorrect voltage signal to the PCM.
• Sticking Internal Oil Pressure Switches (Mazda) (Very Common) — Metallic debris clogs fluid passages in SKYACTIV transmissions, causing internal switches to stick and report incorrect pressures. This is a known issue covered by Mazda TSBs.
• Low, Dirty, or Incorrect Transmission Fluid (Common) — Contaminated or low fluid prevents pressure switches from operating correctly. Using non-OEM fluid alters hydraulic pressure and triggers false readings.
• Damaged Wiring or Loose Connector (Common) — Frayed wires, corrosion, or a loose electrical connector disrupt the 5V reference signal traveling between the switch and the PCM.
• Internal Transmission Mechanical Failure (Less Common) — Worn clutch packs or failing internal seals cause actual hydraulic pressure drops, meaning the switch is accurately reporting a mechanical failure.
• Faulty Powertrain Control Module (PCM) (Rare) — The vehicle's computer fails and misinterprets the sensor signals. This is extremely rare and should only be considered after all electrical tests pass.

Symptoms

• Check Engine Light is On — The universal first warning that the PCM has detected a circuit fault.
• Flashing 'D' or 'D4' Light (Honda/Acura) — The specific transmission fault indicator on the dashboard blinks continuously.
• 'AT' Warning Light On (Mazda) — Illuminates alongside the Check Engine Light to indicate a transaxle-specific fault.
• Harsh, Delayed, or Slipping Shifts — The transmission slams into 2nd gear, or the engine revs unusually high before engaging the gear.
• Stuck in Gear (Limp Mode) — The transmission locks into 2nd or 3rd gear to prevent damage, limiting top speed and causing poor acceleration.

Diagnostic Flowchart

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

Common Fixes & Costs

• Replace 2nd Clutch Pressure Switch (Honda/Acura) — Parts: $30-$80, Labor: $70-$120, ~0.5 hr book time (DIY)
• Replace Internal Oil Pressure Switches / Valve Body (Mazda) — Parts: $700-$1200, Labor: $400-$800, ~4.5 hr book time (Professional)
• Transmission Fluid Drain and Fill — Parts: $50-$120, Labor: $100-$150, ~0.8 hr book time (DIY)
• Repair Damaged Wiring — Parts: $10-$50, Labor: $150-$300, ~2.0 hr book time (Professional)
• Replace Transmission Control Module (TCM) — Parts: $700-$1500, Labor: $100-$250, ~1.5 hr book time (Professional)

Used vs. New Parts: Buying Guide

When a used part is worth it: Buying a used pressure switch is never recommended due to the high failure rate of aged internal diaphragms. Used parts only make sense for complete valve body assemblies if budget is the absolute primary concern.

Donor-vehicle mileage cap: roughly under 50000 miles for the part to have meaningful remaining life.

Donor quality checklist:

• For valve bodies, look for clean fluid and no signs of overheating (discoloration).
• Verify the donor vehicle was not scrapped due to transmission failure.

Decision logic:

• If The part is a pressure switch or sensor under $100 → Always buy a new OEM part. The labor to replace it twice outweighs any savings.
• If The part is a complete valve body and budget is tight → A used part from a low-mileage, rear-ended vehicle is a viable option.

Warranty tradeoff: Salvage yards offer 30-day part warranties but no labor coverage. New OEM parts offer guaranteed fitment and a 1-year warranty.

Worst-case if a used part fails: $150-$400 if a used pressure switch fails after install, requiring repeat labor.

What Happens If You Wait — Timeline

1. 0-1 month: Check Engine Light illuminates. On Hondas, the 'D' light flashes. Shifting feels normal or slightly harsh. The switch is beginning to fail electrically. (MPG impact: 0-2%% · Added cost: $0)
2. 1-3 months: Shifting becomes noticeably harsher. The transmission occasionally enters 'limp mode' but resets after restarting the car. Hydraulic pressure is now inconsistent. (MPG impact: 3-8%% · Added cost: $50-$150 in wasted fuel)
3. 3-6 months: The vehicle frequently enters limp mode. Harsh shifting causes accelerated wear on internal clutch packs. In Mazdas, contaminated fluid damages the valve body. (MPG impact: 10-15%% · Added cost: $500-$1500)
4. 6+ months: Catastrophic failure. Worn clutch material thoroughly contaminates the system. The repair escalates from a $150 sensor replacement to a $3,000+ transmission rebuild. (MPG impact: 15-25%% · Added cost: $3000-$6000)

Cost of Not Fixing It

• 0-1 month: Annoying harsh shifting and flashing warning lights. Expect a 5-10% drop in fuel economy due to inefficient shifting or limp mode. (Added cost: Negligible)
• 1-6 months: Continued harsh shifting increases wear on internal clutch packs. Limp mode forces high engine RPMs, overheating the catalytic converter. (Added cost: $1200-$2800 for catalytic converter replacement.)
• 6+ months: Contaminated fluid from worn clutches clogs passages and solenoids, turning a minor repair into a complete transmission rebuild. (Added cost: $3000-$6000)

Diagnosis Steps

1. Scan for Related Codes
   Use an OBD-II scanner. P1739 (3rd clutch) often pairs with P1738 on Hondas. P0842 or P0780 on Mazdas confirms the internal switch TSB issue.
   Tools: OBD-II Scanner (Beginner)
2. Check Transmission Fluid
   Verify fluid is at the 'FULL' mark, reddish, and smells normal. Low or burnt fluid requires immediate replacement and often resolves intermittent pressure codes.
   Tools: Rag or paper towel (Beginner)
3. Inspect the Switch and Connector
   Locate the 2nd clutch pressure switch. Check for frayed wires, corrosion, or fluid leaking directly from the switch body. Ensure the connector clicks securely into place.
   Tools: Flashlight (Intermediate)
4. Check PCM Reference Voltage
   Turn ignition ON (engine off). Disconnect the switch. Measure the power wire on the harness side. It must show exactly 5V. 0V or 12V indicates a wiring short or open circuit.
   Tools: Multimeter (Advanced)
5. Test Switch Resistance
   Unplug the switch (engine off). Measure resistance across the switch terminal and its metal body. A healthy switch reads infinite resistance (OL). Near zero ohms means it is shorted and dead.
   Tools: Multimeter (Intermediate)
6. Monitor Live Data
   Use an advanced scan tool to watch the 'ATF Pressure Switch 2' PID. It must toggle between 'ON' and 'OFF' as the transmission shifts into 2nd gear. If it stays stuck, the switch or circuit is bad.
   Tools: Advanced OBD-II Scan Tool (Advanced)
7. Perform Dynamic Pressure Test
   Connect a mechanical pressure gauge to the 2nd clutch test port. Compare running pressure to factory specs (typically ~33 PSI for Hondas). Correct pressure plus the code confirms a bad switch. Low pressure confirms internal transmission failure.
   Tools: Transmission pressure gauge set, vehicle-specific service manual (Professional)

When This Code Triggers (Freeze-Frame Conditions)

• Engine Coolant Temp: 180-200°F (The code typically sets when the vehicle is fully warmed up and fluid is at operating viscosity.)
• RPM: 1500-2500 RPM (Occurs during steady-state cruising or light acceleration around the 1-2 or 2-3 shift point.)
• Engine Load: 20-50% (The fault is detected under moderate engine load, not during heavy acceleration.)
• Vehicle Speed: 20-45 mph (Triggered exactly as the transmission is commanded to shift into or out of 2nd gear.)

Related Codes

• P1739 — Indicates a fault in the 3rd Clutch Pressure Switch. It frequently appears alongside P1738 on Hondas. The switches are similar but located in different positions on the transmission.
• P0700 — A generic code meaning the TCM has stored a fault. It always appears alongside P1738. Diagnosing and fixing P1738 automatically clears P0700.
• P0845 — The generic SAE code for 'Transmission Fluid Pressure Sensor/Switch B Circuit Malfunction'. Some scanners display this instead of P1738. Diagnosis is identical.
• P0780 — On Mazdas, P0780 ('Gear Shifting Malfunction') stored with P1738 strongly confirms the root cause is sticking internal oil pressure switches requiring valve body repair.

Climate & Environmental Factors

• Road Salt / Humidity: In regions with heavy road salt use, the external pressure switch connector is highly susceptible to corrosion. This disrupts the voltage signal, triggering P1738 even if the switch is functional.
• Extreme Cold: In very cold weather, transmission fluid thickens. This increased viscosity slows down hydraulic pressure buildup, causing a delay that the TCM interprets as a pressure switch circuit fault.

How to Talk to a Mechanic About This Code

Say this: "For Honda: 'I have a P1738 code. Please test the 2nd clutch pressure switch and its 5V reference wiring.' For Mazda: 'My Mazda has a P1738. Are you familiar with TSB 05-004/19? I want to diagnose it following that procedure, not just quote a new transmission.'"

This proves you researched the code. For Honda, it directs the mechanic to the cheapest fix. For Mazda, it prevents a common misdiagnosis that leads to an unnecessary $5,000 transmission replacement quote.

Avoid saying:

• 'My transmission is slipping, how much to fix it?'
• 'My check engine light is on, can you look at it?'
• 'Just fix whatever is wrong.'

Questions to ask before authorizing the repair:

• For Honda: Did you verify the 5V reference signal at the connector before condemning the switch?
• For Mazda: Are you replacing just the two internal switches or the entire valve body?
• Can you provide a written estimate breaking down parts and labor?
• What is the warranty on this specific repair?

Where to Take It: Dealer vs Independent vs Chain

• Dealer: Highly recommended for Mazda to ensure TSBs are followed. Optional for Honda.
  Best for: Vehicles under powertrain or emissions warranty., Mazda vehicles requiring TSB-specific TCM reprogramming.
  Downsides: Highest labor rates., May default to replacing the whole valve body when just the switches are needed. (Typical cost: +50% vs. baseline)
• Independent Shop: Best fit for Honda/Acura. Good for Mazda ONLY if they confirm they can perform the TSB reprogramming.
  Best for: Out-of-warranty Honda and Acura vehicles., Owners looking for a lower-cost alternative to the dealer.
  Downsides: May lack the specific Mazda M-MDS software for required reprogramming. (Typical cost: +0% vs. baseline)
• Chain Shop: AVOID. The risk of misdiagnosis or upselling a full transmission replacement is too high.
  Best for: Basic services like oil changes.
  Downsides: High pressure to upsell services., Lacks specific diagnostic expertise for internal transmission codes. (Typical cost: -10% vs. baseline)

When to Walk Away From the Repair

If the estimated repair cost exceeds 50% of the car's private-party value, strongly consider selling the car as-is.

• Car worth $4000, fix is $200: Fix it. This is a minor repair on a Honda/Acura and well below the threshold.
• Car worth $6000, fix is $2200: Borderline. This is a typical cost for the Mazda valve body repair. Get a second opinion before proceeding.
• Car worth $3000, fix is $2400: Walk away. The repair cost is nearly the entire value of the car.

What Scan Tool You Need for This Code

Minimum: An OBD-II scanner that reads manufacturer-specific transmission codes and views live data PIDs.

A $20 code reader cannot display the live on/off status of the pressure switch, which is required to test the circuit. It also cannot read the secondary Mazda codes (P0842) that confirm the TSB.

Budget: BlueDriver Pro (~$100) — Reads transmission codes and displays live data for the pressure switch status on most vehicles.

Mid-range: Foxwell NT510 Elite (~$180) — Provides OE-level diagnostics. Reads all transmission codes, views live data, and performs system-specific tests.

Professional: Autel MaxiCOM MK808S (~$500) — Offers full-system diagnostics, live data graphing, and bidirectional controls for transmission relearn procedures.

Rent vs buy: For a one-time diagnosis on a Honda, renting a scanner from AutoZone is sufficient. For Mazda issues, buying the BlueDriver or Foxwell is a worthwhile investment.

How to Clear the Code After You Fix It

1. Use an OBD-II scan tool to clear the diagnostic trouble codes (DTCs).
2. Perform a specific drive cycle to allow the vehicle's readiness monitors to run.
3. Check for pending codes again after the drive cycle.

Drive cycle (~30 minutes): Cold start and idle for 3 minutes. Drive at a steady 50-60 mph for 10 minutes. Perform mixed city driving with several complete stops and moderate accelerations. Allow the vehicle to cool completely.

Readiness monitors affected: Comprehensive Component Monitor, Catalyst Monitor

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

Watch out for:

• Disconnecting the battery clears the code but resets all readiness monitors, causing an automatic emissions test failure.
• The code returns immediately if the 5V reference wire is still broken.
• Not performing a varied enough drive cycle leaves monitors 'Not Ready'.

Will This Fail Emissions / State Inspection?

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

• California: An illuminated Check Engine Light is an automatic failure. After repair, a specific drive cycle must be completed to set all readiness monitors before re-testing.
• New York: Any active transmission code that illuminates the Check Engine Light results in an automatic failure during the OBD-II scan.
• Texas: An active P1738 code is an automatic fail. If recently cleared, the vehicle fails if too many readiness monitors read 'Not Ready'.

Most Commonly Affected Vehicles

• Honda Accord (1998-2007) — Extremely common on 6th and 7th generation Accords. The external 2nd clutch pressure switch frequently fails. OEM part 28600-RKE-004.
• Acura TL (1999-2003) — Shares the Accord's transmission design and is highly susceptible to the same external pressure switch failures.
• Honda Odyssey (1999-2004) — The 2nd clutch pressure switch is a frequent failure point on these minivans, often accompanied by a flashing 'D' light.
• Honda Pilot (2003-2005) — Early Pilot models experience this code due to a faulty external 2nd clutch pressure switch.
• Acura CL (1998-2003) — Frequent complaint pointing to the 2nd clutch pressure switch. V6 models typically use part 28600-P7W-013.
• Mazda CX-5, CX-3, CX-9, Mazda3, Mazda6 (2012-2021) — P1738 means 'Automatic Transaxle Internal Malfunction' and points to sticking internal oil pressure switches in the valve body. Covered by TSB 05-004/19.
• Hyundai Terracan, Tucson (2004-2006) — Indicates a 'Shift system timeout' related to the transfer case control unit (TCCU), completely separate from the main transmission.
• BMW 3 Series / 5 Series (E36, E46, E39) (1996-2006) — Indicates an electrical fault in a pressure control solenoid circuit, often related to the VANOS system rather than the transmission.

Manufacturer-Specific Notes

• Honda / Acura: This code almost exclusively points to the external 2nd clutch pressure switch. A flashing 'D' light is the classic symptom. The fix is a straightforward, cheap sensor replacement.
• Mazda: On SKYACTIV models, P1738 means 'Automatic Transaxle Internal Malfunction' and points to sticking internal oil pressure switches. The fix requires TCM reprogramming and internal valve body work.
• Hyundai: On 4WD models like the Terracan, P1738 indicates a 'Shift system timeout' related to the transfer case control unit, completely separate from the main transmission.
• Jaguar: P1738 means 'Shift Time Error', indicating the transmission is not completing shifts within the expected time due to internal hydraulic issues.
• Volkswagen: On DSG transmissions, P1738 relates to a 'Clutch Position Sensor 2' circuit fault inside the complex mechatronic unit.

Real Owner Stories

2000 Honda Accord (233k miles) - Easy Fix

Check Engine Light came on. Owner had recently changed the transmission fluid and had the transmission rebuilt 30,000 miles prior.

What they tried:

1. Verified transmission fluid level was correct.
2. Scanned the code, which returned P1738.
3. Replaced the external 2nd clutch pressure switch.

Outcome: Replacing the pressure switch turned off the Check Engine Light and resolved the code immediately.

Lesson: On older Hondas, P1738 is almost always a failed external pressure switch. It's an inexpensive part that can be replaced in 30 minutes without further diagnostics.

2014 Mazda CX-5 - A Story of Misdiagnosis

Check Engine Light and 'AT' warning light came on. A dealership quoted $9,000 for a complete transmission replacement.

What they tried:

1. The owner sought a second opinion at an independent shop.
2. The shop referenced Mazda TSB 05-002-23, which describes this exact issue.

Outcome: The shop reprogrammed the TCM and replaced the two internal oil pressure switches on the valve body for $1,400, permanently resolving the code.

Lesson: Never agree to a full transmission replacement on a Mazda for this code without ensuring the shop has followed the specific TSB for internal switch replacement.

1999 Honda Accord - Code Appears After New Transmission Install

A P1738 code appeared just a few months after a new transmission was installed.

What they tried:

1. The owner took the car to AutoZone to pull the code.
2. Inspected the 2nd clutch pressure switch and its connector.

Outcome: The connector was loose and contaminated with fluid from the recent install. Cleaning and reseating the connector fixed the issue.

Lesson: If a code appears shortly after a major repair, check the external sensors and connectors first. They are easily disturbed during heavy mechanical work.

How to Prevent This Code From Triggering

• Perform regular transmission fluid drain-and-fills (Every 30,000-60,000 miles) — Clean fluid prevents the buildup of clutch material and metallic debris. This contamination is the primary cause of sticking internal switches in Mazda models.
• Use only OEM or equivalent specified transmission fluid (Every fluid change) — Manufacturer-specific fluids (Honda ATF-DW1 or Mazda ATF-FZ) have precise frictional properties. Generic fluid causes poor shifting and premature component wear.
• Come to a complete stop before shifting from Drive to Reverse (Daily habit) — Shifting while moving puts immense stress on transmission bands and clutches, dramatically accelerating mechanical wear and fluid contamination.
• Replace the external transmission filter (if applicable) (Every 60,000 miles) — On Honda/Acura models with an in-line filter, replacing it removes contaminants, protecting sensitive solenoids and pressure switches from debris.

Frequently Asked Questions

Can I fix a P1738 code myself?

Yes, on Honda and Acura models, replacing the external 2nd clutch pressure switch is a simple DIY job requiring basic hand tools. Mazda models, however, require professional internal valve body work.

What happens if I ignore the P1738 code?

Ignoring P1738 causes worsening shift quality and forces the car into limp mode. This accelerates internal clutch wear and overheats the catalytic converter, leading to thousands in repairs.

How much does it cost to fix P1738 in 2024?

A Honda/Acura pressure switch replacement costs $100-$200 at a shop. Mazda valve body repairs range from $1,100 to $2,400 depending on labor rates.

Is a P1738 code a sign of a bad transmission?

Rarely on Hondas, where it usually indicates a cheap external sensor failure. On Mazdas, it points to internal valve body contamination, which requires significant repair but not always a full transmission replacement.

Why is my 'D' light flashing along with the Check Engine Light?

On Honda and Acura vehicles, a flashing 'D' light is the dedicated transmission warning indicator. It means the transmission control module has detected a specific fault like P1738.

I replaced the switch, but the code came back. Now what?

Verify you used an OEM-quality switch and check the wiring harness for hidden breaks or corrosion. If the 5V reference signal is missing at the connector, the fault lies in the wiring or the PCM.

What is a common misdiagnosis for P1738?

Mechanics often confuse the 2nd clutch switch (P1738) with the 3rd clutch switch (P1739) and replace the wrong one. Another costly mistake is quoting a full transmission replacement for a Mazda without checking the specific TSB for internal switches.

Can a weak battery cause a P1738 code?

Yes. Low system voltage causes the PCM to misinterpret sensor signals, triggering false transmission codes. Always test your battery and alternator before replacing transmission parts.

Key Takeaways

• On 1998-2007 Honda and Acura vehicles, P1738 almost exclusively points to a failed external 2nd clutch pressure switch, which costs under $80 to replace yourself.
• For 2012-2021 Mazda SKYACTIV models, this code indicates sticking internal oil pressure switches requiring a $1,200+ valve body repair outlined in TSB 05-004/19.
• Stop driving if the transmission enters 'limp mode' (locked in 2nd or 3rd gear), as the resulting high RPMs will destroy your catalytic converter and cost upwards of $2,500.
• Always verify the 5V reference signal at the switch connector with a multimeter before buying parts; 0V means you have a wiring short, not a bad sensor.