P0690
ECM/PCM Power Relay Sense Circuit High
Powertrain Network/Communication ECM Power Supply 🟡 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 receiving the correct voltage signal it expects, like a light switch reporting it's on when it should be off. This voltage sensing circuit is stuck at a higher level than normal, preventing proper power management.

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Symptoms You May Notice
3 known symptoms for this code
Engine cranks but won't start or starts intermittently
Electrical systems behave erratically or lose power unexpectedly
Dashboard warning lights flicker or stay illuminated
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Embedded Systems Insight
What the ECU/ECM is actually computing

The ECM monitors the power relay sense circuit voltage to verify the relay is switching properly and supplying stable power. It expects a voltage drop when the relay de-energizes and a rise when it energizes. A continuously high voltage indicates the relay sense line is stuck or the relay feedback circuit is open.

Voltage & Parameter Thresholds

Parameter Normal Range Fault Condition
Relay Sense Voltage 0.5V to 3.5V (cycling with relay state) Above 4.5V continuously or no voltage drop detected
Relay Response Time 50-200 milliseconds No state change or excessive delay
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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 main ECM power connectors with a wire brush.
2
Power relay connectors
Inspect and reseat the main power relay connector in the engine bay fuse/relay box.
3
Main power relay
Replace the ECM/PCM power relay if voltage remains high after connector checks.