P0687
ECM/PCM Power Relay Control Circuit High
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's power relay control circuit is stuck in a high voltage state, meaning the relay isn't responding properly to the computer's commands. Think of it like a light switch that's jammed in the 'on' position—the switch won't turn off when told to.

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Symptoms You May Notice
3 known symptoms for this code
Engine stalls or won't start
Dashboard warning lights illuminate
Electrical system malfunction or loss of power
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Embedded Systems Insight
What the ECU/ECM is actually computing

The ECM monitors the voltage level on the power relay control circuit to verify the relay is responding to commands. When the ECM grounds the relay coil, voltage should drop to near 0V; if voltage remains abnormally high, it indicates a circuit fault. The ECM sets this code when the measured voltage exceeds the normal operating threshold.

Voltage & Parameter Thresholds

Parameter Normal Range Fault Condition
Relay Control Voltage 0.5–1.0V when energized >4.5V (stuck high)
Command Response Time <100ms No voltage drop detected
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Diagnostic & DIY Fix Guide
Check these in order — from cheapest to most complex
1
Wiring harness and connectors
Inspect and clean the relay connector pins and wiring for corrosion or loose connections.
2
Power relay
Test the relay with a multimeter; replace if it fails the continuity or resistance test.
3
ECM/PCM software or module
Reflash the ECM or replace the module if wiring and relay tests pass.