What Does P0688 Mean?
The PCM has detected an open circuit on the ECM/PCM power relay sense circuit. The sense circuit is a feedback wire that tells the PCM whether the main power relay has actually engaged. Without this feedback, the PCM cannot confirm its own power supply is stable. The driver may experience intermittent no-start conditions, random stalling, or warning lights flickering.
Safety Warning
The engine may stall without warning if the PCM interprets the missing sense signal as a power loss and initiates a shutdown. Drive only if absolutely necessary and avoid highway travel until the issue is resolved.
Common Causes
35%
Open wire in the relay sense/feedback circuit between relay and PCM
25%
ECM/PCM power relay contact failure (contacts not closing)
20%
Corroded or backed-out pin at PCM connector for sense circuit
10%
Blown fuse in the relay output circuit feeding the sense wire
10%
Relay socket contact corrosion causing open at the sense terminal
Diagnostic Steps
1
With the key on and relay engaged, measure voltage at the PCM sense circuit input pin; it should show battery voltage if the relay is engaged and the sense wire is intact.
2
Check for continuity of the sense wire from the relay output terminal to the PCM connector pin; an open reading confirms a broken wire.
3
Inspect the PCM connector for backed-out or corroded pins on the sense circuit terminal; use the manufacturer's pin chart to identify the correct pin.
4
Verify the relay output contacts are closing by measuring voltage on the relay output terminal with the key on; no voltage means the relay contacts are open.
5
Check all fuses downstream of the relay that may also feed the sense circuit; a blown fuse can remove the sense signal.
Estimated Repair Cost
$50 - $450
Parts + labor, varies by vehicle and location
The PCM has detected an open circuit on the ECM/PCM power relay sense circuit. The sense circuit is a feedback wire that tells the PCM whether the main power relay has actually engaged. Without this feedback, the PCM cannot confirm its own power supply is stable. The driver may experience intermitte...
The most common cause of P0688 (ECM/PCM Power Relay Sense Circuit Open) is: Open wire in the relay sense/feedback circuit between relay and PCM
Typical repair costs for P0688 range from $50 to $450, depending on the vehicle, location, and whether you do it yourself or go to a shop.
The engine may stall without warning if the PCM interprets the missing sense signal as a power loss and initiates a shutdown. Drive only if absolutely necessary and avoid highway travel until the issue is resolved.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to P0688 to identify the root cause.
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Quick Info
Category
Powertrain
System
Engine Control Module Power Supply
Difficulty
Type
Generic (SAE)
Recommended Tools
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