How to Fix P0685 Code

The P0685 Code indicates a problem with the ECM/PCM power relay control circuit, often causing starting issues or stalling. This guide walks you through diagnosing the fault, inspecting wiring and battery connections, and replacing faulty components to clear the code and restore reliable engine performance.

Key Takeaways

• Understand the code: P0685 points to an open or high‑resistance condition in the ECM/PCM power relay circuit.
• Check the basics first: Inspect battery terminals, grounds, and fuses before moving to more complex components.
• Test the relay: Swap or measure the PCM power relay; a faulty relay is the most common culprit.
• Examine wiring: Look for corroded connectors, chafed harnesses, or loose pins that can interrupt power.
• Replace or reprogram the ECM/PCM only if needed: Most cases are fixed by repairing the relay or wiring.
• Prevent recurrence: Keep battery connections clean, use dielectric grease, and schedule regular electrical system checks.

Understanding the P0685 Code

The P0685 Code is a generic OBD‑II trouble code that signals a fault in the ECM/PCM power relay control circuit. When the engine control module (ECM) or powertrain control module (PCM) does not receive the expected voltage from its power relay, it stores this code and may illuminate the check‑engine light. While the code itself does not point to a specific component, it tells you that the power supply to the engine’s computer is compromised.

Typical symptoms include difficulty starting the engine, intermittent stalling, or the vehicle refusing to crank at times not turning over at all. In some cases, the engine may start but run rough, and you might notice a loss of power under load. Because the ECM/PCM manages fuel injection, ignition timing, and many other vital functions, any interruption in its power can lead to drivability problems.

Understanding that P0685 is essentially a “power‑relay‑circuit” issue helps you focus your diagnostic steps. Rather than guessing at sensors or emission components, you’ll concentrate on the relay, its wiring, the battery supply, and the ground paths that feed the computer.

Diagnosing the P0685 Code

Before you start replacing parts, a systematic diagnosis saves time and money. You’ll need a few basic tools: a digital multimeter (DMT), a test light or logic probe, a set of basic hand tools, and a vehicle‑specific wiring diagram (often found in the service manual or online).

Step‑by‑step diagnostic process

1. Verify the code: Connect an OBD‑II scanner, read the stored codes, and note any accompanying codes (e.g., P0686, P0687). Clear the codes and see if P0685 returns after a key cycle.
2. Check battery voltage: With the engine off, measure voltage at the battery terminals. You should see 12.6 V or higher. Low voltage can trigger false codes.
3. Inspect the PCM power relay: Locate the relay (usually in the power distribution box under the hood). Refer to the wiring diagram to identify pin 30 (constant battery power), pin 85 (ground), pin 86 (ECM/PCM trigger), and pin 87 (switched output to the ECM/PCM).
4. Test the relay: Using the DMT, check for continuity between pins 85 and 86 (coil) – you should see low resistance (around 70 Ω). Then, with the key in the ON position, verify that pin 86 receives a ground signal from the ECM/PCM. Finally, confirm that pin 30 has constant battery voltage and that pin 87 outputs voltage when the relay is energized.
5. Examine wiring and connectors: Follow the harness from the relay to the ECM/PCM. Look for signs of corrosion, broken pins, chafed insulation, or loose connectors. Pay special attention to any area where the harness passes near hot exhaust or moving parts.
6. Check grounds: Locate the ECM/PCM ground points (often on the engine block or chassis). Clean them, ensure they are tight, and measure resistance to the battery negative – it should be near zero ohms.
7. Inspect fuses: Verify that any fuse protecting the PCM power relay circuit is intact. A blown fuse will produce an open circuit and trigger P0685.

If the relay tests good, voltage is present at the battery and grounds, and the wiring appears intact, the fault may lie within the ECM/PCM itself. However, in the majority of cases, the issue is found in the relay, its connections, or the battery supply.

Common Causes of P0685

Understanding the typical failure points helps you prioritize your inspection. Below are the most frequent culprits behind a P0685 Code.

Faulty PCM power relay

The relay is an electromechanical switch that can wear out over time. Internally, the contacts may pit or the coil may develop an open circuit, preventing the ECM/PCM from receiving power when the ignition is turned on.

Wiring issues and corroded connectors

Automotive wiring is subject to vibration, heat, and moisture. Over years, connectors can corrode, especially if dielectric grease was not applied. A high‑resistance connection mimics an open circuit, triggering the code.

Battery and grounding problems

A weak battery, loose terminals, or corroded cable ends can drop the voltage supplied to the relay. Similarly, a poor ground path for the ECM/PCM will cause the control module to see insufficient voltage, setting P0685. Keeping your battery terminals clean and tight is essential – you can learn more about proper terminal maintenance in our guides on how to fix a loose battery terminal, securing battery terminals, and cleaning battery corrosion.

Blown fuse or faulty fuse box

Sometimes a fuse that protects the relay circuit blows due to a short or overload. Replacing the fuse without addressing the underlying short will cause the code to return quickly.

ECM/PCM failure

Although rare, internal failure of the engine control module can prevent it from energizing the relay coil or reading the feedback voltage. This is usually diagnosed only after all external causes have been ruled out.

How to Fix P0685 Code

Once you’ve identified the root cause, the repair is straightforward. Follow these steps to clear the code and restore reliable operation.

Replace the PCM power relay

If the relay test shows high coil resistance, no continuity, or the contacts are burned, replace it with an OEM‑spec or high‑quality aftermarket relay. Ensure the new relay is seated firmly in the socket and that the retaining clip (if present) is engaged.

Repair wiring and connectors

For any corroded or damaged connectors:

• Disconnect the battery negative terminal.
• Unplug the connector and inspect both sides.
• Clean male and female pins with electrical contact cleaner and a small brush.
• If pins are bent, straighten them carefully with needle‑nose pliers.
• Apply a thin layer of dielectric grease to prevent future corrosion.
• Reconnect and secure any locking tabs.

If you find a chafed wire, wrap the damaged section with electrical tape or, better yet, replace the harness segment using solder and heat‑shrink tubing for a durable repair.

Clean and tighten battery terminals

Loose or corroded battery terminals are a frequent source of intermittent power loss. Follow these steps:

1. Disconnect the negative cable first, then the positive.
2. Use a battery terminal cleaner or a mixture of baking soda and water to scrub away corrosion.
3. Rinse with clean water and dry thoroughly.
4. Reattach the cables, ensuring the clamps bite firmly onto the terminals.
5. Apply a light coat of petroleum jelly or terminal protector spray to inhibit future corrosion.

This simple maintenance often resolves P0685 when the root cause is a poor battery connection.

Check and replace fuses

Locate the fuse that protects the PCM power relay circuit (refer to your owner’s manual or fuse box diagram). If the fuse is blown, replace it with one of the same amperage. After replacement, re‑scan for codes; if the fuse blows again, you have a short that needs further tracing.

ECM/PCM reprogramming or replacement

If all external checks pass and the code persists, the ECM/PCM may be at fault. In many modern vehicles, a software glitch can cause the module to misinterpret relay feedback. A dealership or qualified shop can reflash the module with the latest calibration. Should the module be defective, replacement is the final option—though this is costly and should be considered only after exhaustive diagnostics.

Preventive Measures and Maintenance Tips

Keeping the electrical system healthy reduces the chance of seeing P0685 again.

Regular battery maintenance

Check battery voltage monthly, clean terminals every 6 months, and ensure the hold‑down clamp is tight. A healthy battery provides stable voltage to all vehicle computers.

Inspect wiring harnesses

During routine oil changes or tire rotations, take a quick look at visible wiring under the hood. Look for signs of wear, rodent damage, or moisture intrusion. Early detection prevents costly repairs later.

Use quality parts

When replacing relays, fuses, or connectors, choose OEM‑spec or reputable aftermarket brands. Cheap components may fail prematurely, leading to repeat codes.

Schedule periodic electrical system scans

Even if the check‑engine light is off, an annual OBD‑II scan can catch pending codes before they become noticeable problems.

When to Seek Professional Help

While many P0685 fixes are DIY‑friendly, some situations warrant a mechanic’s expertise.

Signs you need a mechanic

• The code returns immediately after you clear it, despite checking the relay, wiring, and battery.
• You notice multiple related codes (e.g., P0686, P0687, P0688) indicating a broader power‑distribution issue.
• The engine cranks but won’t start, and you suspect a faulty ECM/PCM.
• You lack the tools or confidence to safely test high‑current circuits.

Cost considerations

Replacing a PCM power relay typically costs $15‑$40 for the part and minimal labor if you do it yourself. Wiring repairs vary but usually stay under $100 if you need to replace a short harness segment. ECM/PCM replacement or reprogramming can run $500‑$1500, depending on the vehicle make and model. Getting a professional diagnosis first can save you from unnecessary part replacement.

Conclusion

The P0685 Code may seem intimidating at first, but it is fundamentally a power‑supply problem for your vehicle’s engine computer. By methodically checking the battery, terminals, fuses, relay, and wiring, you can resolve the majority of cases without expensive component replacement. Regular maintenance—especially keeping battery connections clean and tight—goes a long way toward preventing this code from reappearing. If you’ve walked through the steps and the light stays on, don’t hesitate to consult a qualified technician who can perform advanced diagnostics or ECM/PCM reprogramming. With the right approach, you’ll have your car running smoothly and the check‑engine light off in no time.

Frequently Asked Questions

What does the P0685 Code mean?

The P0685 Code indicates an open or high‑resistance condition in the ECM/PCM power relay control circuit, meaning the engine computer is not receiving the proper voltage it needs to operate.

Can a loose battery terminal cause P0685?

Yes. A loose or corroded battery terminal can drop the voltage supplied to the PCM power relay, triggering the P0685 Code. Cleaning and tightening the terminals often resolves the issue.

How do I test the PCM power relay?

With the key in the ON position, use a multimeter to verify constant battery voltage at relay pin 30, a ground signal at pin 86 from the ECM/PCM, and switched output voltage at pin 87 when the relay is energized. Also check the coil resistance between pins 85 and 86.

Is it safe to drive with a P0685 Code?

Driving with the code present is risky because the ECM/PCM may lose power unexpectedly, leading to stalling or failure to start. It’s best to diagnose and repair the issue promptly.

What tools do I need to diagnose P0685?

You’ll need a digital multimeter, basic hand tools, a test light or logic probe, and a wiring diagram for your vehicle’s power distribution and ECM/PCM circuits.

How much does it typically cost to fix a P0685 Code?

If the fix is a relay, wiring, or battery terminal service, expect $20‑$150. If the ECM/PCM requires replacement or reprogramming, costs can rise to $500‑$1500 depending on the vehicle.