P0685 Code Meaning
The engine cranks and never catches, or it starts and then goes silent a second later. The scanner stores P0685. The description says ECM/PCM power relay control circuit, which is easy to read as “the computer is bad.”
The code is about the circuit that turns the main power relay on, not a blanket processor failure. On many vehicles that relay is the ASD or auto-shutdown relay. On others it is simply the ECM power relay. Jeep and Dodge use this circuit a lot, but the same generic code shows up on other makes too.
This blog covers what P0685 means, the usual relay, fuse, and wiring causes, and how to tell those apart from a module that can no longer drive the relay.
P0685 Code Meaning Quick Answer
P0685 means ECM/PCM Power Relay Control Circuit. The computer commands the main power relay, then watches the control side. If the command and the result do not match, it sets P0685. You may also see a no-start, a stall right after startup, or no injector or coil power.
The relay, its fuse, and the wiring fail more often than the module that controls them. A failed PCM can still be the cause, but only after the relay circuit has been tested. If the engine fires and then dies, that pattern overlaps with our dies after starting guide.
Common Causes of a P0685 Code
Start at the relay and work toward the computer. That order keeps a $20 part from turning into a module guess.
1. A Failed ASD or Main Power Relay
The ASD or ECM power relay is what actually feeds injectors, coils, and sometimes the module’s switched power. When the contacts burn or the coil opens, the PCM can command the relay and get nothing back. That mismatch is P0685.
Swap in a known-good relay of the same type, or have the shop command it with a scan tool and listen for the click. On Chrysler products this is a regular failure, but Ford, GM, and others use a similar main relay under a different name.
2. A Blown Fuse
A fuse on the relay output or the control feed can open and leave the circuit looking dead to the PCM. The engine may crank with no injector pulse, which is the same symptom we cover in no injector pulse while cranking.
Check the ASD, ECU, and ignition fuses with a meter, not just by eye. An open fuse is a clue, not the whole repair. Find the short or overloaded circuit that opened it.
3. Wiring or Connector Damage
The control wire from the PCM to the relay coil, and the sense circuit that reports relay status, both have to be intact. A chafed harness, a corroded socket, or a pin that backed out can set P0685 with a perfectly good relay and a good computer.
Trace the relay control circuit, tug-test the connectors, and look for rodent damage near the battery and the under-hood distribution center. Repair the wire before you replace either end of it.
4. A Bad Ground or Low Voltage
If the relay coil or the PCM cannot see a solid ground, the control circuit fails the monitor even though the parts are still capable. Low battery voltage can do the same thing during cranking, when the system sags and the relay chatters.
Load-test the battery and clean the grounds at the engine, body, and module. Then clear P0685 and see if it returns on a full-voltage crank.
5. A Failed PCM or ECM
Less often, the driver inside the module that grounds the relay coil has failed. The relay never clicks, the fuse and wiring are good, and power never reaches the injectors. At that point the computer is the fault, not the relay.
Confirm power and ground at the module, then test whether it can actually command the relay. If you need a broader map of related faults, use the car computer codes guide before you order a replacement.
How to Tell P0685 Causes Apart
Listen for the relay and measure the circuit. The sound and the voltage usually name the cause.
| Cause | How to tell | What to do |
|---|---|---|
| Failed ASD or main relay | No click when the key is turned to run, and a known-good relay restores power | Replace the relay and recheck the circuit that overheated it |
| Blown fuse | The relay output or control fuse is open | Replace the fuse only after finding the short or overload |
| Wiring or connector damage | The relay and fuse are good, but the control or sense wire is open or shorted | Repair the harness and corroded pins |
| Bad ground or low voltage | The battery sags while cranking, or grounds are corroded | Charge or replace the battery and clean grounds |
| Failed PCM or ECM | Relay, fuse, wiring, and voltage all test good and the module never commands the relay | Replace the PCM or ECM after confirming the driver is dead |
Fix the Relay Circuit First
P0685 is a circuit code. Treat the relay, fuse, wiring, and voltage as the job unless those pieces already passed. Jeep and Dodge owners see this often because the ASD circuit is busy, but the same logic applies on other brands. Browse Jeep or Dodge modules only after the relay side is cleared.
Car Computer Exchange supplies programmed replacements when the PCM can no longer control the power relay.
Call 1-888-875-2958 or email sales@carcomputerexchange.com once testing points to the module.
Frequently Asked Questions
What does the P0685 code mean?
P0685 means ECM/PCM Power Relay Control Circuit. The computer commands the main power relay and monitors that circuit. If the command and the result do not match, it stores P0685. The cause can be the relay, a fuse, wiring, low voltage, or a failed module.
Can a bad ASD relay cause P0685?
Yes. On many Jeep, Dodge, and Chrysler vehicles the ASD relay is the main power relay in that circuit. Burned contacts or an open coil can set P0685 and leave the engine with no injector or coil power. Other makes use a similar relay under a different name.
Will a car start with a P0685 code?
Sometimes it will crank and never start. Sometimes it starts and dies as soon as the relay drops out. If the relay never closes, there is often no spark and no injector pulse. Do not keep cranking it until the circuit is tested.
Does P0685 mean I need a new PCM?
Not by itself. P0685 is more often a relay, fuse, wiring, or voltage problem. Replace the PCM or ECM only after the relay circuit tests good and the module still will not command or sense the relay correctly.
How do I fix a P0685 code?
Test the ASD or main power relay, the related fuses, the control and sense wiring, and system voltage. Repair what failed, clear the code, and retest. If those items are good and P0685 returns, have the PCM or ECM tested and replaced if the relay driver is dead.
