P0689
ECM/PCM Power Relay Sense Circuit Low
Powertrain Network/Communication Power Management & Relay Control 🟡 Moderate — Fix within a week
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What This Actually Means
In plain language — no jargon

The engine control computer detects a weak or missing electrical signal from the power relay that supplies it with voltage. Think of it like a doorbell that's not ringing loud enough—the main switch (relay) isn't delivering the expected power signal back to the control unit.

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Symptoms You May Notice
3 known symptoms for this code
Engine won't start or stalls immediately after starting
Instrument cluster lights flicker or dim erratically
Check Engine Light illuminates with no other obvious issues
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Embedded Systems Insight
What the ECU/ECM is actually computing

The ECM monitors the feedback voltage from the main power relay contact to verify it's delivering adequate voltage. This sense circuit confirms the relay is functioning and supplying power. If the voltage drops below the expected threshold, the ECM logs a fault because it cannot trust the power delivery system.

Voltage & Parameter Thresholds

Parameter Normal Range Fault Condition
Power Relay Sense Voltage 10.5–14.5 volts (engine running) Below 4.5 volts or open circuit detected
Sense Circuit Continuity Continuous signal with <0.5 volt drop Intermittent or missing signal
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Diagnostic & DIY Fix Guide
Check these in order — from cheapest to most complex
1
Battery terminal connectors and cable
Clean corrosion from battery posts and tighten all connections, then clear the code.
2
Main power relay (under-hood fuse box)
Locate the relay, remove and reseat it firmly into its socket to restore contact.
3
Wiring harness and connectors at ECM/PCM
Inspect the sense circuit wire for damage, corrosion, or loose terminals and repair as needed.