Enter the error code and car brand

Vehicle engine and/or automatic transmission

Code: P0685 - The control circuit for the ECM/PCM (Engine/Powertrain Control Module) power relay is open, meaning the relay may not be supplying power properly.

Let me break down what p0685 really means, straight from years in the shop. When you see this code pop up, it’s telling you the car’s main computer-the PCM or ECM-just isn’t getting the juice it needs. Why? The power relay, which acts like an electronic gatekeeper for the computer’s power supply, isn’t closing the circuit when it should. No power to the brain of your car, and suddenly nothing works right. Now, different brands have their own way of wiring these relays, but it all comes down to one thing: your computer needs to wake up and do its job the second you turn the key. If anything blocks that relay from feeding power, you’ll be staring at a p0685 code before you know it. I’ve seen this play out way too many times-don’t underestimate how vital that relay circuit is to everything under the hood.

Causes and P0685 Trouble Code

In my experience, when a p0685 comes in, nine times out of ten it boils down to a few usual suspects:

A tired or faulty PCM power relay. These little guys can stick, get fried, or simply wear out after years of cycling on and off.
Wiring or connector issues. I’m talking corroded pins, loose terminals, or wires that have rubbed through their insulation-especially between the relay, ignition switch, and PCM. I’ve chased down a lot of headaches caused by nothing more than a hidden green fuzz on a connector.
Sometimes, though rarely, you’re looking at a bad PCM itself. Internal faults in the computer can keep it from talking to or controlling the relay like it should.

Most of the time, you’ll find the problem with the relay or the wiring, not the actual computer. If you’re working on a Honda or any other make, start simple and save yourself a world of trouble. Don’t jump straight to the PCM-check the basics first.

Symptoms and OBD2 Code P0685

Wondering what you’ll notice if p0685 pops up? Here’s what usually walks into my bay:

The check engine light is lit up-that’s often your first (and sometimes only) warning.
Either the engine cranks but won’t fire, or you get nothing at all when you turn the key.
Hard starting is a classic, especially when it’s cold outside. I’ve seen plenty of customers stuck in the driveway first thing in the morning, scratching their heads.

When the PCM isn’t powered, your car’s just not going to cooperate. Ignore these red flags and you’re asking for a tow truck. Believe me, your car’s trying to tell you something important here.

Diagnosis and P0685 Engine Code

When I get a car in with a P0685, here’s how I roll up my sleeves and get to work:

Start at square one-check the battery and the main fuses. You want to see solid voltage and no blown fuses hiding in plain sight.
Track down the PCM power relay (your owner’s manual or a wiring diagram will point you in the right direction). I always look for burn marks, corrosion, or anything that looks off. Here’s a pro-tip: swap in a known good relay or just wiggle the one you’ve got-sometimes that’s all it takes to spot a flaky connection.
Next up, inspect every inch of wiring and the connectors running between the relay, ignition switch, and PCM. I’m hunting for frayed wires or any sign of corrosion-grab a flashlight and really dig in. If you’ve got a helper, have them turn the key while you check voltage at the relay’s terminals. It’s amazing what you’ll find just by poking around.
If everything’s looking good so far, I bring out the multimeter and test the relay control circuit. With the key on, you should have battery voltage at the relay coil and a solid ground on the other side. Missing either? Time to backtrack and find out where the circuit’s breaking down.
Only after I’ve ruled out relays and wiring do I look at the PCM as the culprit. It’s rare, but sometimes the computer just can’t do its job anymore. Double-check everything before you go ordering a new brain for your car-it’s expensive and usually not necessary.

One last piece of advice: always check for Technical Service Bulletins (TSBs) for your make and model. Sometimes there’s a factory fix or updated part that can save you hours of frustration.

Common Mistakes with P0685

Here’s a classic pitfall I see all too often-folks rush out and buy a new PCM without ever checking the relay or wiring. That’s a fast way to empty your wallet for no reason. Another slip-up is checking only the power side of the relay and forgetting about ground-a relay needs both to work. And don’t get me started on dirty connectors: corrosion inside those little pins can throw you for a loop. Always, always start with the basics. Check the easy stuff before tearing into the complicated bits.

Seriousness of dtc P0685

Let me be blunt: dtc p0685 isn’t something you can afford to ignore. When your car’s computer loses power, you could wind up stranded without warning. Worse still, the engine might quit while you’re driving-never a good scenario. I’ve seen people keep driving and end up with a dead battery, or even a fried PCM if there’s a short in the system. The stakes are high here: you risk the PCM, the relay, and sometimes other control modules that share that power circuit. Don’t wait-fix this before it turns into an expensive nightmare.

Repair Steps for P0685 Code

Here’s my tried-and-true approach to getting rid of p0685:

Swap out the PCM power relay if it’s sticking, burned, or just acting up. Relays are cheap and easy to replace.
Patch up or replace any sketchy-looking wiring or connectors between the relay, ignition switch, and PCM. Don’t settle for electrical tape-do it right.
Scrape off any corrosion on terminals, and make sure everything is tight and snug. Loose connections love to cause trouble.
If, and only if, you’re absolutely sure the PCM is fried after checking everything else, go ahead and replace or reprogram it. But that’s your last resort, not your first move.

Always start with the simple stuff-relays and connectors-before you open your wallet for something expensive. This advice holds true whether you’ve got dtc p0685 honda, Ford, Toyota, you name it.

Conclusion

Bottom line-p0685 means your car’s computer isn’t getting the power it needs thanks to a relay or wiring problem. This isn’t a code to brush off; you could be left stranded or end up with a much bigger repair bill if you ignore it. From my time under the hood, I can tell you: start with the relay and wiring every single time. Don’t even think about the PCM until you’ve ruled out everything else. Get on top of this quickly, and you’ll keep your ride safe and dependable.

Video for "Error p0685" on YouTube