P0685
ECM/PCM Power Relay Control Circuit/Open
Powertrain Network/Communication ECM Power Control 🟡 Moderate — Fix within a week
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What This Actually Means
In plain language — no jargon

The ECM/PCM power relay isn't responding to control signals from the engine computer, like a light switch that won't turn on the power supply. This prevents the computer from managing engine startup and operation.

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Symptoms You May Notice
3 known symptoms for this code
Engine won't start or cranks but won't fire
No dashboard lights or instrument cluster activity
Check Engine Light illuminated
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Embedded Systems Insight
What the ECU/ECM is actually computing

The ECM monitors voltage and continuity in the power relay control circuit, which should show proper signal voltage when the ignition is turned on. The ECM expects to detect a valid ground path and control signal to activate the relay that supplies power to critical engine systems. If the circuit is open or the relay doesn't respond to the control signal, a fault is triggered.

Voltage & Parameter Thresholds

Parameter Normal Range Fault Condition
Relay Control Signal Voltage 12V or valid PWM signal present 0V or open circuit detected
Relay Coil Ground Path Continuous path with <1Ω resistance Open circuit or >5Ω resistance
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Diagnostic & DIY Fix Guide
Check these in order — from cheapest to most complex
1
Battery terminals and connectors
Clean corrosion from battery posts and engine bay connectors to restore proper electrical contact.
2
Power relay (fuel pump or main relay)
Test relay with multimeter or swap with known-good relay of same specification to confirm operation.
3
Wiring harness and connectors
Inspect relay circuit wiring for damage, loose connectors, or broken pins and repair or replace as needed.