OBD-II Code P0685: ECM/PCM Power Relay Control Circuit / Open

What Does P0685 Mean?

P0685 means the Engine Control Module (ECM) or Powertrain Control Module (PCM) detects an open circuit in its main power supply. A dedicated relay acts as a gate, supplying the voltage required for the module to manage fuel injection and ignition timing. Without this stable power supply, the vehicle's computer cannot operate, resulting in a no-start condition or sudden engine stalling.

Technical definition: The SAE/OBD-II definition for P0685 is "ECM/PCM Power Relay Control Circuit / Open". The PCM monitors the voltage on the control circuit of the relay that supplies its battery voltage. If the PCM detects 0 volts (an open circuit) when the relay is off, or fails to receive power after commanding the relay on, it sets this code. It also triggers if voltage exceeds 4.6 volts when the key is on.

Can I Drive With P0685?

No — Do Not Drive. Do not drive with this code. The engine will stall at any moment without warning, creating a major safety risk at high speeds or in traffic. Continuing to drive causes further electrical system damage or forces the vehicle into a severely restricted 'limp mode'.

Common Causes

• Faulty PCM/ECM Power Relay (Very Common) — The power relay is a small electronic switch that wears out, gets stuck, or fails internally from age, heat, or vibration. This is the most frequent cause of a P0685 code.
• Blown Fuse or Fusible Link (Common) — A fuse protecting the PCM or its relay circuit can blow. Fuses usually blow because of an underlying short circuit or sudden voltage spike that must be addressed.
• Corroded or Loose Battery Terminals/Cables (Common) — Corrosion or a loose connection at the battery posts prevents the entire electrical system from receiving steady voltage. Cables can also corrode internally, creating high resistance even if the terminals look clean.
• Damaged or Corroded Wiring/Connectors (Common) — Wires in the relay circuit fray, break, or short due to heat, vibration, or rodent damage. The relay's socket or the PCM's connectors also corrode in moisture-prone areas, blocking electrical flow.
• Bad Ground Connection (Less Common) — The PCM relies on a solid ground connection to function. A loose, corroded, or broken ground strap for the engine or chassis mimics an open in the power relay circuit.
• Faulty Under-hood Fuse Box / Junction Block (TIPM) (Less Common) — Internal corrosion or a cracked circuit board within the fuse box assembly creates an open circuit. This is especially common in Chrysler/Jeep vehicles equipped with a Totally Integrated Power Module (TIPM).
• Faulty Powertrain Control Module (PCM) (Rare) — An internal failure in the PCM's power management circuit triggers this code. All other possibilities must be ruled out first due to the high cost of replacement.
• Improper 'After-Run' Power Down Sequence (Rare) — On specific vehicles (like Hyundai and Honda), the PCM performs a shutdown sequence after the key turns off. If the power relay de-energizes too quickly or too slowly, the PCM logs a fault.

Symptoms

• Engine will not start — The engine cranks but will not fire because the PCM lacks power to control the fuel injectors and ignition coils.
• Engine stalls unexpectedly — The engine starts and runs before suddenly shutting off while driving. This happens when the PCM relay loses power intermittently.
• Check Engine Light is on — The Malfunction Indicator Lamp (MIL) illuminates on the dashboard to alert the driver of the fault.
• Hard starting in cold weather — The vehicle requires extended cranking to start, caused by low voltage or an intermittent connection in the PCM power circuit.
• Vehicle enters 'Limp Mode' — To protect the engine, the vehicle enters a reduced power mode, severely limiting acceleration and speed.
• No communication with scan tool (also visible on scanner) — An OBD-II scanner cannot communicate with the PCM because the module has no power. The scanner screen may flicker or fail to connect.

Common Fixes & Costs

• Replace the PCM Power Relay — Parts: $15-$50, Labor: $0-$50, ~0.2 hr book time (DIY)
• Replace Blown ECM/PCM Fuse — Parts: $2-$10, Labor: $0-$25, ~0.1 hr book time (DIY)
• Clean or Replace Battery Terminals/Cables — Parts: $5-$50, Labor: $25-$100, ~0.8 hr book time (DIY)
• Repair Damaged Wiring or Connectors — Parts: $10-$50, Labor: $100-$300, ~1.5 hr book time (Intermediate)
• Replace Under-hood Fuse Box (TIPM) — Parts: $400-$800, Labor: $150-$300 (plus programming), ~1.5 hr book time (Professional)
• Replace the Powertrain Control Module (PCM) — Parts: $500-$1,200, Labor: $150-$300 (includes programming), ~1.2 hr book time (Professional)

Used vs. New Parts: Buying Guide

When a used part is worth it: For expensive electronic modules like a PCM or TIPM on a high-mileage vehicle, a remanufactured part offers significant savings over a new OEM part.

Donor-vehicle mileage cap: roughly under 100000 miles for the part to have meaningful remaining life.

Donor quality checklist:

• Verify the part number on the used module matches your original part exactly.
• Source parts from dry climates to avoid internal corrosion issues.
• Ensure the seller offers a warranty and return policy.

Decision logic:

• If The faulty part is a simple relay or fuse → Always buy new. The cost is low ($5-$50) and reliability is paramount.
• If The faulty part is a TIPM or PCM and the vehicle is over 150K miles → A remanufactured unit with a warranty offers the best balance of cost and reliability. A junkyard part is the cheapest but riskiest option.
• If The vehicle is newer or has low mileage → Favor a new OEM part to ensure full compatibility and avoid programming headaches.

Warranty tradeoff: Junkyard parts have a 30-90 day warranty. Remanufactured modules offer a 1-year to lifetime warranty. New OEM parts carry a 1-year manufacturer's warranty.

Worst-case if a used part fails: $400-$1200 if a used electronic module fails, including repeat diagnostics, programming labor, and sourcing another part.

Cost of Not Fixing It

• Immediate: Sudden, unexpected engine stalling while driving. This is a severe safety hazard that leads to accidents in traffic or at highway speeds. (Added cost: $500+ (Towing, potential accident damages))
• 0-1 month: Intermittent no-start conditions leave you stranded. Repeated cranking with low voltage strains the starter motor and battery. (Added cost: $200-$600 (Cost of a new battery and/or starter motor))
• 1-6 months: Chronic low voltage or intermittent power spikes corrupt PCM software or damage sensitive internal circuits, destroying the computer. (Added cost: $800-$1500+ (Cost of PCM/TIPM replacement and programming))

Diagnosis Steps

1. Perform a Relay Swap Test
   Find another relay in the fuse box with the exact same part number that controls a non-essential system (like the horn). Swap the suspected bad PCM relay with the identical good one. If the car starts, the original relay is bad.
   Tools: None (pliers optional) (Beginner)
2. Inspect the Battery and Terminals
   Ensure the battery is fully charged (12.4-12.6V at rest). Verify the terminals are clean, tight, and free of greenish-white corrosion. A weak battery is a primary trigger for this code.
   Tools: Wrench set, battery terminal cleaner, digital multimeter (Beginner)
3. Check the Fuses
   Locate the fuse box and find the fuse labeled for the ECM, PCM, or Main Relay. Pull the fuse and inspect the internal metal strip. If broken, replace it with a new fuse of the exact same amperage.
   Tools: Fuse puller, owner's manual (Beginner)
4. Locate and Inspect the PCM Relay Socket
   Pull the PCM power relay straight out and inspect the metal prongs and the socket for corrosion, discoloration, or melted plastic indicating heat damage.
   Tools: Owner's manual, flashlight (Beginner)
5. Test the Relay Socket for Power
   With the relay removed and ignition ON, use a multimeter to check for voltage at the socket terminals. You should find one terminal with constant 12V power from the battery (Pin 30) and another that gets 12V when the key is 'ON' (Pin 86). No power indicates a wiring or fuse problem upstream.
   Tools: Digital multimeter (Intermediate)
6. Check for a Ground Signal from the PCM
   The PCM provides a ground signal to activate the relay (Pin 85). Check the relay socket terminal going to the PCM; when the key is on, it should show a ground connection (near 0V). No ground indicates a wiring issue or internal PCM fault.
   Tools: Digital multimeter (Advanced)
7. Check Relay Coil Resistance
   Set your multimeter to Ohms (Ω). Measure the resistance between the relay's control circuit pins (Pin 85 and 86). A good relay reads between 50 and 200 Ohms. 'OL' indicates an open coil, and near zero indicates a shorted coil.
   Tools: Digital multimeter (Advanced)
8. Bench Test the Relay Function
   Check continuity between the power input (Pin 30) and output (Pin 87); it should read 'OL'. Apply 12V to pin 86 and ground to pin 85. The relay should click, and continuity between pins 30 and 87 should drop to near zero Ohms (less than 1 Ω).
   Tools: Digital multimeter, 12V power source (Advanced)
9. [PRO TIP] Perform a Voltage Drop Test
   Connect the positive multimeter lead to the battery's positive post and the negative lead to the PCM relay's power supply pin (Pin 30) while cranking the engine. A reading above 0.2V indicates excessive resistance. Repeat on the ground side; readings above 0.1V indicate a bad ground.
   Tools: Digital multimeter with long leads (Advanced)
10. [PRO TIP] Analyze Live Scan Tool Data
    Navigate to the live data stream for the engine control module. Select PIDs for 'ECM/PCM Relay Voltage' or 'Module Input Voltage'. With the key ON, voltage should be 12.0-12.6V. A reading below 9V confirms a power delivery issue.
    Tools: Advanced OBD-II Scan Tool with Live Data (Advanced)

When This Code Triggers (Freeze-Frame Conditions)

• Engine State: Key-On, Engine-Off or Cranking (The fault is detected during the initial power-up sequence before the engine starts, or during cranking when voltage stability is critical.)
• Module Voltage: < 9.0 Volts (Freeze frame data shows a module voltage significantly lower than battery voltage, indicating a power delivery failure.)
• Engine RPM: 0 or erratic (If the fault occurs while driving, RPM drops to zero at the moment of the stall. If it's a no-start, RPM shows 0 or a low cranking speed.)
• Vehicle Speed: 0 mph (Since the code is most often set during startup or results in a stall, the vehicle is typically stationary when the fault is logged.)

Related Codes

• P0686 — Indicates the PCM relay control circuit voltage is too low. It often appears with P0685 and points to a weak battery or high resistance.
• P0687 — Indicates the PCM relay control circuit voltage is too high, suggesting a short to power.
• P0688 — Refers to the 'sense' circuit of the PCM power relay, used to verify the relay's switched power side is working.
• P0562 — Means 'System Voltage Low.' It frequently appears alongside P0685 because a weak battery causes low system voltage, triggering the relay fault.

Climate & Environmental Factors

• High Humidity / Heavy Rain: Accelerates corrosion on battery terminals, ground connections, and within the fuse box/TIPM. Moisture seeping into the relay socket creates high resistance or short circuits.
• Cold Weather: Low temperatures drop a weak battery's voltage below the threshold the PCM requires. Intermittent connections from cracked solder joints separate as metal contracts, causing a no-start.
• Hot Weather / High Under-Hood Temps: Heat is a primary cause of failure for relays. For Honda PGM-FI relays, heat causes internal solder joints to crack, leading to stalling after the engine warms up.

How to Clear the Code After You Fix It

1. Reconnect the battery if it was disconnected for the repair.
2. Use a compatible OBD-II scan tool to erase the P0685 trouble code from the PCM's memory.
3. Start the engine and let it idle for at least 30 seconds to allow the PCM to re-establish normal operations.

Drive cycle (~20 minutes): Perform a drive cycle to ensure the fix is complete: a cold start, idling for 2-3 minutes, 10-15 minutes of mixed city/highway driving, followed by shutting the engine off and letting it cool.

Readiness monitors affected: Comprehensive Component Monitor, Misfire 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 underlying electrical fault (bad relay, wiring, fuse) is not repaired.
• Not driving the vehicle long enough leaves readiness monitors in a 'Not Ready' state.

Will This Fail Emissions / State Inspection?

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

• California: An illuminated Check Engine Light from a P0685 code is an automatic failure. All OBD readiness monitors must be set to 'Ready', requiring a 50-100 mile drive cycle after repair.
• New York: The NYS DMV emissions inspection includes an OBD-II scan. A P0685 code causes an automatic test failure. The issue must be repaired and cleared before passing.
• Texas: In emissions-testing counties, an active P0685 code results in an automatic failure. You cannot pass the state inspection until the problem is fixed.

Most Commonly Affected Vehicles

• Ford F-150, Focus, Transit (2005-2020) — Prone to intermittent no-start conditions caused by a failing or sticking ECM relay. On some models, the Body Control Module (BCM) sends a 'wake up' signal to the PCM, adding a step to diagnostics.
• Honda Accord, Odyssey (2005-2017) — Frequently experiences this code due to a failing PGM-FI main relay, a common wear item. Also prone to issues caused by aftermarket stereo installations creating a parasitic draw.
• Jeep Grand Cherokee, Wrangler (2011-2021) — This code is frequently a symptom of a failing Totally Integrated Power Module (TIPM). The internal fuel pump relay fails, causing a P0685 and a no-start condition.
• Volkswagen Jetta, Golf, Passat (2005-2020) — A known issue exists where the power wire to the ECM becomes damaged within the wiring harness, about 6 to 18 inches from the computer's connector.
• Chevrolet Silverado, Malibu (2013-2016) — Experiences P0685 due to wiring issues near the fuse box or internal PCM failure related to water intrusion shorting the module.
• BMW 3 Series, 5 Series, X3, X5 (2007-2017) — Commonly triggered by a failing main relay (often a blue relay in the E-box) or issues with the integrated power module.
• Hyundai Santa Fe, i20, Elantra (2011-2020) — This code sets if the main relay deactivates too quickly or too slowly after key-off, as the PCM monitors a specific 'after-run' shutdown process.
• Subaru Impreza, Forester, Outback (2010-2018) — Often related to a failing main relay or corrosion in the main fuse box located in the engine bay.

Manufacturer-Specific Notes

• Ford: On later models, the code sets if the power relay remains energized after the key turns off. The computer expects to go to sleep and sets the code when it stays powered up due to a stuck relay.
• Volkswagen/Audi: For 2.5L engines (2005-2007), a well-documented TSB addresses a broken Red/Black power wire inside the harness, 6 to 18 inches from the ECM connector, directly causing P0685.
• Honda: P0685 is frequently caused by a failing PGM-FI main relay due to cracked internal solder joints. A safety recall (07-020) was issued for fuel pump relays made by 'MITSUBA' on 2005-2006 models.
• Hyundai/Kia: The PCM monitors the timing of the main relay's de-energization. If the relay turns off too soon or remains on for more than 15 seconds after key-off, the PCM sets P0685 on the next cycle.
• Jeep/Chrysler/Dodge: Many models use a Totally Integrated Power Module (TIPM) where internal relays are soldered to a circuit board. A P0685 code often requires the entire TIPM to be replaced and programmed.

Frequently Asked Questions

Can a bad battery cause a P0685 code?

Yes. A weak or failing battery is a very common cause. If voltage drops too low during cranking, the PCM detects a fault in its power relay circuit and triggers the code.

Can a bad ground cause P0685?

Yes. The PCM and its relay require a solid ground to complete the circuit. A loose or corroded engine ground strap creates high resistance, preventing the relay from activating properly.

What is a relay swap and can I do it myself?

A relay swap is a simple diagnostic trick where you exchange the suspect relay with an identical one from a less critical circuit, like the horn. It requires no tools and quickly confirms if the relay is the problem.

What are the most common misdiagnosis mistakes for P0685?

The biggest mistake is immediately replacing the PCM, which is expensive and rarely the root cause. Always rule out the battery, fuses, relay, and wiring first. A thorough voltage and ground check of the relay circuit is mandatory before condemning the computer.

I replaced the relay and the code came back. What's next?

If a new relay fails to fix the code, inspect the associated fuses, battery terminals, and relay socket for corrosion. Use a multimeter to confirm 12V power and ground at the correct pins in the socket. The problem lies in the electrical path, not the relay itself.

My car starts sometimes but not others. Could this still be P0685?

Yes, an intermittent no-start is a classic symptom of a P0685 fault. It points to a failing relay or a loose connection affected by vibration or temperature changes.

Is it expensive to fix code P0685?

It ranges from very cheap to very expensive. A simple fuse or bad relay costs under $50, while a wiring repair runs $100-$300. If the PCM or TIPM has failed, the repair exceeds $1,000 after parts and programming.

What does the ECM power relay do?

The ECM power relay is an electrically operated switch that controls when the engine computer gets power from the battery. It uses a small current from the ignition switch to close a high-current circuit, powering up the vehicle's brain.

Key Takeaways

• A faulty $20 PCM power relay causes over 70% of P0685 codes, making a simple relay swap the best first diagnostic step.
• Never drive with an active P0685 code; the engine will stall unpredictably at highway speeds, causing a total loss of power steering and braking assist.
• Verify battery voltage is at least 12.4V and inspect the main ECM fuse before condemning expensive modules like the PCM or TIPM.
• Perform a voltage drop test on the relay circuit before replacing the PCM; readings above 0.2V on the power side or 0.1V on the ground side indicate a wiring fault, not a bad computer.