Seeing a check engine light can be worrying, especially when the code points to a critical system like your car’s computer. If your scanner shows P0685, you’re dealing with a specific electrical issue. The P0685 Honda trouble code indicates an ECM/PCM Power Relay Control Circuit Malfunction, meaning the circuit that controls the main power to your engine computer isn’t working right.
This problem can prevent your Honda from starting or cause it to stall unexpectedly. Don’t panic, though. This guide will explain what the code means, why it happens, and how you can fix it. We’ll walk you through the diagnosis step-by-step, from simple checks to more complex solutions.
P0685 Honda Trouble Code: ECM/PCM Power Relay Control Circuit Malfunction
Let’s break down what this technical description actually means. The ECM (Engine Control Module) or PCM (Powertrain Control Module) is your car’s main computer. It needs constant power to run your engine properly.
The ECM/PCM power relay is a switch that provides this main power. The “control circuit” is the wiring and signals that tell the relay when to turn on and off. A “malfunction” in this circuit means the computer is not getting the command to power up correctly, or it’s detecting an fault in the signal path.
Common Symptoms of the P0685 Code
You might notice several signs before or after the check engine light comes on. Here are the most common symptoms:
- No-Start Condition: The most severe symptom. The engine may crank but will not start at all because the ECM has no power.
- Engine Stalling: The car may start but then stall immediately or stall while driving if the relay cuts power.
- Check Engine Light: Obviously, the P0685 code will illuminate the MIL. It may be the only code or appear with others.
- Loss of Power: The engine might run but feel sluggish or unresponsive due to intermittent computer power.
- Dead Dashboard: In some cases, you might see other electrical gremlins, like gauges not working or warning lights behaving oddly.
What Causes the P0685 Code in a Honda?
Several issues can trigger this code. It’s usually related to the relay, its wiring, or the computer itself. Here’s a list of the typical culprits:
- Faulty ECM/PCM Power Relay: This is the most common cause. The relay itself fails internally.
- Blown Fuse: A fuse protecting the relay control circuit can blow, interrupting power.
- Bad Wiring or Connections: Corroded, damaged, or loose wires in the control circuit can cause a malfunction. This includes connectors at the relay, fuse box, or ECM.
- Failed Engine Control Module (ECM/PCM): Although less common, the computer itself could have an internal fault preventing it from controlling the relay.
- Low Battery Voltage: A weak or dying battery can cause low voltage signals that the ECM interprets as a circuit fault.
- Problems with the Ignition Switch: The signal to activate the relay often comes from the ignition switch circuit.
How to Diagnose and Fix P0685: A Step-by-Step Guide
Safety first! Always disconnect the negative battery terminal before working on electrical components. Have your repair manual handy for specific fuse and relay locations for your Honda model.
Step 1: Preliminary Checks
Start with the simple and free checks. These can often save you alot of time and money.
- Check the Battery: Ensure the battery terminals are clean and tight. Test the battery voltage. It should be at least 12.4 volts with the engine off.
- Scan for Other Codes: Use your OBD2 scanner to check for any additional trouble codes. Write them down, as they can provide clues.
- Visual Inspection: Look for any obvious damage to wiring around the relay box and the main engine harness. Look for chewed wires or corrosion.
Step 2: Locate and Test the ECM/PCM Power Relay
Find the main relay. In most Hondas, it’s located under the dashboard on the driver’s side, in the under-hood fuse/relay box, or a combination of both. Consult your manual.
- Once located, try swapping it with an identical relay from another slot (like the horn or A/C relay).
- Clear the code and see if it returns. If the code moves or doesn’t come back, you’ve found the problem.
- For a more precise test, you can remove the relay and use a multimeter to check for coil resistance and switch continuity. A failed relay will show infinite resistance or no continuity.
Step 3: Check the Related Fuses
Find the fuse diagram. Check every fuse related to the ECM, PCM, and fuel system. This includes the “ECM” fuse, “IG Coil” fuse, and “PGM-FI” main relay fuse. A visual inspection is good, but use a multimeter to check for continuity to be sure the fuse isn’t blown.
Step 4: Inspect Wiring and Connectors
This step requires careful attention. You’re looking for the control wires between the ECM, the relay, and the ignition switch.
- Disconnect the battery.
- Inspect the wiring harness for chafing, burns, or cuts. Pay special attention where the harness passes through the firewall or near sharp edges.
- Unplug connectors at the relay, fuse box, and ECM. Look for bent pins, green corrosion, or moisture. Clean them with electrical contact cleaner if needed.
- Use a multimeter to check for continuity in the control circuit wires. Also check for shorts to ground or power.
Step 5: Check for Power and Ground Signals
With the battery reconnected and the ignition in the “ON” position, use your multimeter. Back-probe the relay socket (carefully) to verify it is receiving power and a ground signal from the ECM when the key is turned. If power is present but the control signal from the ECM is missing, the issue may be further upstream.
Step 6: Consider the ECM/PCM Itself
This is a last resort due to cost. If all wiring, fuses, and the relay check out perfectly, the computer itself could be faulty. Professional diagnosis with advanced scan tools that can monitor the relay control signal is recommended before replacing the ECM. Sometimes, a software update from the dealer can resolve a control glitch.
Estimated Repair Costs
Costs can vary widely based on the cause and your location.
- ECM/PCM Power Relay: $20 – $80 for the part. It’s often a quick, DIY fix.
- Wiring Repair: $100 – $300 if a professional needs to trace and repair a broken wire.
- ECM/PCM Replacement: $500 – $1,500+. This includes the part (new or refurbished) and programming/programming at a shop.
- Diagnostic Fee: A mechanic typically charges $100 – $150 for diagnosis, which may be waived if you have them do the repair.
Can You Drive with the P0685 Code?
It is not recommended. The problem can lead to a sudden engine stall, leaving you stranded or causing a dangerous situation in traffic. If the car starts and runs, you might be tempted to drive it to a shop, but do so with extreme caution and avoid busy roads. The safest choice is to have it towed.
Preventing Future Electrical Issues
Good maintenance can help avoid problems like P0685. Keep your battery terminals clean and secure. Address any minor electrical issues immediately before they lead to bigger ones. Also, when replacing fuses or relays, use quality parts to ensure reliability.
FAQ Section
What does the P0685 code mean on a Honda Civic?
On a Honda Civic, P0685 has the same meaning: ECM/PCM Power Relay Control Circuit Malfunction. The diagnosis process is very similar to other Honda models.
Is the main relay the same as the ECM power relay?
Yes, in most Hondas, the terms “main relay,” “PGM-FI main relay,” and “ECM/PCM power relay” refer to the same component. It’s responsible for supplying power to the fuel pump and the engine computer.
Can a bad battery cause a P0685?
Yes, a weak battery can cause low voltage conditions that the ECM misinterprets as a circuit malfunction. Always test your battery and charging system first when dealing with electrical trouble codes.
Where is the ECM relay located?
The location varies by model and year. Common places include the under-hood fuse/relay box (often labeled “Main” or “PGM-FI”) or an interior fuse panel under the dashboard on the driver’s side. Check your owner’s manual for the exact spot.
How do I reset the P0685 code?
You can clear it with an OBD2 scanner after the repair is made. Disconnecting the battery for several minutes may also clear it, but this will also reset other memory in the car. The code will likely return immediately if the underlying problem isn’t fixed.
Dealing with a P0685 code requires a methodical approach. Start with the simple, inexpensive fixes like the relay and fuses before moving to complex wiring or computer diagnosis. While it can be a frustrating issue, understanding the circuit helps you tackle it with confidence. If you’re ever unsure, consulting a trusted mechanic is always a smart move to get your Honda running smoothly again.