ECM/PCM Power Relay Sense Circuit Low
P0689 indicates the PCM/ECM has detected abnormally low voltage in the power relay sense circuit, which monitors the relay's output voltage to the engine control module. This circuit is critical for proper PCM operation and power management; if voltage drops below the expected threshold, the PCM triggers this code to alert you of a potential electrical power delivery problem.
What You Might Notice
- Delayed or difficult engine starting
- Engine fails to start at all
- Electrical accessories not functioning properly
- Engine drivability issues or rough running
- Dashboard warning lights and gauges behaving erratically
Most Common Causes
- 1
Defective PCM Power Relay
The power relay that supplies voltage to the PCM may be failing or not engaging properly, reducing voltage in the sense circuit. This is the most common cause of P0689.
- 2
Blown Fuse or Fusible Link
A fuse or fusible link in the PCM power supply circuit may be blown, interrupting voltage to the relay sense circuit. Check the underhood and interior fuse boxes for damaged fuses.
- 3
Open or Shorted Wiring Circuit
An open connection or short circuit between the power relay and the PCM sense line can cause voltage to drop below specifications. Corrosion, damaged insulation, or loose connectors are common culprits.
- 4
Poor Electrical Connections
Corroded or loose battery terminals, relay connectors, or PCM connectors can reduce voltage delivery to the sense circuit. Battery voltage issues or weak battery capacity can also contribute.
How to Diagnose It
- 1
Check Defective PCM Power Relay — The power relay that supplies voltage to the PCM may be failing or not engaging properly, reducing voltage in the sense circuit. This is the most common cause of P0689.
- 2
Check Blown Fuse or Fusible Link — A fuse or fusible link in the PCM power supply circuit may be blown, interrupting voltage to the relay sense circuit. Check the underhood and interior fuse boxes for damaged fuses.
- 3
Check Open or Shorted Wiring Circuit — An open connection or short circuit between the power relay and the PCM sense line can cause voltage to drop below specifications. Corrosion, damaged insulation, or loose connectors are common culprits.
- 4
Check Poor Electrical Connections — Corroded or loose battery terminals, relay connectors, or PCM connectors can reduce voltage delivery to the sense circuit. Battery voltage issues or weak battery capacity can also contribute.
How to Fix It
- 1
Replace PCM Power Relay
Locate the PCM power relay (typically in the engine bay fuse/relay box) and replace it with an OEM or quality replacement. Test voltage at the relay output after installation to confirm proper operation.
- 2
Inspect and Replace Blown Fuses
Check the main fuse box and any in-line fusible links related to PCM power. Replace any blown fuses with the correct amperage rating and inspect for signs of electrical shorts.
- 3
Repair Wiring and Connections
Inspect all wiring between the power relay and PCM for cuts, corrosion, or loose connections. Clean corroded connectors with electrical contact cleaner and crimp or solder any damaged wiring back to proper specifications.
- 4
Test Battery and Charging System
Use a voltmeter to verify the battery is holding at least 12.6V at rest and 13.5V while running. If voltage is low, test the alternator output and replace the battery or alternator if defective.
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