P0689
ECM/PCM Power Relay Sense Circuit LowP0689 is a generic OBD-II powertrain diagnostic trouble code: ECM/PCM Power Relay Sense Circuit Low. It is logged by the engine control unit when the glow monitor detects that a specific fault threshold has been exceeded — typically resulting in the malfunction-indicator lamp (MIL / check-engine light) being illuminated.
What P0689 means
P0689 — "ECM/PCM Power Relay Sense Circuit Low" — is set when the PCM detects that the voltage on its own relay feedback (sense) pin is lower than expected after the main power relay should have closed. Unlike P0685, which concerns the relay coil driver output, P0689 concerns the feedback side: a dedicated sense wire that runs from the relay output (the contact side) back to the PCM, allowing the module to confirm that battery voltage has actually reached its supply pins through the relay contacts. When the sense pin reads below the calibrated low-voltage threshold — typically below approximately 10 V — P0689 is stored.
In practice, the sense line being "low" means the relay contacts are not fully conducting: a relay with pitted or burned contacts may still make intermittent electrical contact, allowing the engine to start but causing the sense voltage to dip under load or vibration. Corroded wiring or connectors between the relay output terminal and the PCM sense pin introduce resistance that drops the voltage at the PCM end. The symptoms often present as intermittent stalling on deceleration, unexplained loss of electrical accessories, or an engine that starts and runs but then cuts out without warning — all consistent with the PCM occasionally losing its supply voltage confirmation.
Diagnosis of P0689 begins with the relay and its connector, which are the most common failure points, then proceeds to the sense wiring before the PCM itself is implicated. Because the sense circuit is a direct sample of the relay contact output, a low sense reading always means insufficient voltage is reaching that line — whether due to the relay, the fuse, or the wiring — rather than a PCM calibration issue.
Common causes
Most-frequently reported root causes when P0689 is logged.
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1
ECM/PCM main power relay with pitted, burnt, or high-resistance contacts that fail to deliver full battery voltage through to the sense circuit.
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2
Corroded or loose relay socket terminals or the main harness connector at the relay causing a resistance drop on the output side.
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3
Broken, chafed, or high-resistance wire in the sense line between the relay output terminal and the PCM sense pin.
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4
Blown or partially blown (high-resistance) fuse in the relay output circuit reducing voltage on the sense line.
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5
Loose or corroded battery terminals or cables causing the supply voltage at the relay to drop below the PCM's sense threshold under load.
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6
Short to ground on the sense wire between the relay and the PCM pulling the sense pin voltage low.
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7
Internal PCM fault where the sense input circuit misreads an otherwise valid supply voltage — uncommon but possible after moisture ingress.
Symptoms drivers notice
How to diagnose P0689
A typical diagnostic flow when this code is present.
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1
Connect a scan tool, retrieve all stored DTCs and freeze frame data, and check for companion code P0685 — the presence of both codes together strongly suggests a relay or connector fault rather than a sense-wire-only issue.
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2
With the ignition on (engine off), probe the ECM/PCM sense input pin at the PCM connector with a multimeter; voltage should be within 12.0–13.5 V when the relay is closed — a reading below this confirms the sense circuit is low.
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3
Inspect the ECM/PCM main power relay and socket for burnt contacts, corrosion, or melted plastic; substitute a known-good relay of the same part number and retest.
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4
Check the fuse(s) in the relay output circuit for continuity and verify voltage on both sides of the fuse with the ignition on.
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5
Test continuity and voltage drop along the sense wire from the relay output terminal to the PCM connector; a drop greater than 0.5 V under load indicates a resistance fault in the wiring.
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6
Inspect and clean all ground connections for the relay, PCM, and engine block; a high-resistance ground increases the effective resistance of the entire supply circuit and can drag the sense voltage low.
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7
If relay, fuses, wiring, and grounds all test good and sense voltage remains low at the PCM connector, consult manufacturer-specific test procedures for the PCM sense input circuit before authorising module replacement.
Related powertrain codes
Frequently asked questions
What is the difference between P0685 and P0689?
P0685 is a fault in the relay coil control circuit — the PCM cannot properly command the relay to close. P0689 is a fault on the relay output sense circuit — the relay may be operating, but the voltage reaching the PCM's feedback pin is too low. Both point to the relay and its connector as the first inspection target, but they represent different legs of the circuit.
Why does my car stall while driving but restart after a few minutes?
This is a classic symptom of intermittent relay contact failure. When the relay contacts are cold they may pass enough current to keep the sense voltage acceptable; as they heat under load they expand slightly and resistance increases, dropping the sense line below the PCM threshold. After a brief cool-down the contacts shrink back and the problem temporarily disappears. Relay replacement is the correct fix.
Can a low battery cause P0689?
Yes. If the battery or its cables cannot maintain adequate voltage under cranking or accessory load, the relay output and sense line will drop below the PCM's minimum threshold even if the relay itself is healthy. Always check battery state of charge and cable condition before condemning the relay or PCM.
How do I confirm the relay contacts are the problem and not the sense wire?
Measure voltage directly at the relay output terminal (contact side) with the relay closed. If voltage is normal there but low at the PCM sense pin, the sense wire or its connectors have excessive resistance. If voltage is low at the relay output terminal itself, the relay contacts or the supply fuse are the fault.
Disabling P0689 in software
RaceTune can permanently disable P0689 — and any other OBD-II diagnostic trouble code — on every ECU family we support. The monitor is disabled inside the ECU itself, so the fault stops being logged: the warning light stays off and the engine never enters limp mode for this code. The change is tied to your exact software version.
Software modifications affect emissions compliance and are not road-legal in many jurisdictions. RaceTune service files are intended for motorsport, off-road, and export use.
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