What Does P0685 Mean?
The PCM has detected an open circuit in the ECM/PCM power relay control circuit. The power relay is responsible for providing main power to the engine control module. If this relay fails to engage, the PCM may lose power intermittently or completely, causing a no-start condition or sudden engine stall. The driver may experience intermittent no-start, sudden engine shutdown, or multiple warning lights illuminating simultaneously.
Safety Warning
This is a serious condition. If the PCM power relay fails while driving, the engine will stall without warning, resulting in loss of power steering and reduced brake assist. The vehicle should not be driven until this is repaired. If the engine does run, it may stall unpredictably in traffic.
Common Causes
35%
Failed ECM/PCM main power relay (open coil or stuck contacts)
25%
Open circuit in relay control wire from PCM to relay coil
20%
Blown fuse in the relay control or power feed circuit
15%
Corroded or loose connector at relay socket or PCM
5%
PCM relay driver circuit failure
Diagnostic Steps
1
Locate the ECM/PCM main power relay (typically in the underhood fuse box) and swap it with an identical relay from a non-critical circuit to test if the relay itself is faulty.
2
Measure the relay coil resistance (typically 60-100 ohms); OL indicates an open coil. Also check for continuity across the relay contacts when manually activated.
3
Check the relay control circuit for continuity from the PCM relay driver pin to the relay coil terminal; an open circuit here prevents the relay from energizing.
4
Verify all fuses in the PCM power feed circuit are intact, including any fusible links at the battery junction box.
5
Inspect the relay socket terminals for corrosion, heat damage, or spread terminals that would cause intermittent contact.
6
If the relay and wiring are good, check for proper ground at the relay coil ground terminal; measure voltage drop under load.
Estimated Repair Cost
$50 - $500
Parts + labor, varies by vehicle and location
The PCM has detected an open circuit in the ECM/PCM power relay control circuit. The power relay is responsible for providing main power to the engine control module. If this relay fails to engage, the PCM may lose power intermittently or completely, causing a no-start condition or sudden engine sta...
The most common cause of P0685 (ECM/PCM Power Relay Control Circuit Open) is: Failed ECM/PCM main power relay (open coil or stuck contacts)
Typical repair costs for P0685 range from $50 to $500, depending on the vehicle, location, and whether you do it yourself or go to a shop.
This is a serious condition. If the PCM power relay fails while driving, the engine will stall without warning, resulting in loss of power steering and reduced brake assist. The vehicle should not be driven until this is repaired. If the engine does run, it may stall unpredictably in traffic.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to P0685 to identify the root cause.
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Quick Info
Category
Powertrain
System
Engine Control Module Power Supply
Difficulty
Type
Generic (SAE)
Recommended Tools
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