P0689 — Ecmpcm Power Relay Sense Circuit Low

Generic OBD-II · all makesPowertrain (engine & transmission)

OBD-II code P0689 indicates that the Engine Control Module (ECM) or Powertrain Control Module (PCM) has detected a low voltage condition on the sense circuit that monitors the output of the ECM/PCM power relay. This sense circuit allows the computer to verify that the relay is delivering proper voltage when commanded on. A low reading typically points to a problem in the relay, its power supply, or the wiring between the relay and the ECM.

Symptoms

Engine cranks but does not start
Intermittent stalling or no-start condition
Check Engine Light illuminated
Other modules may report loss of communication with ECM/PCM
Engine may start and run normally if the fault is intermittent
No noticeable symptoms other than the stored code

What causes it — most likely first

1. Faulty ECM/PCM power relay

The relay contacts may be pitted, burned, or sticking, causing high resistance or intermittent open circuit, which drops voltage on the sense line. This is the most common cause.

2. Blown or corroded ECM/PCM power relay fuse

A blown fuse or one with corroded terminals can interrupt power to the relay coil or load circuit, leading to low sense voltage.

3. Damaged or corroded wiring/connectors in the sense circuit

Chafed, broken, or corroded wires between the relay output and the ECM sense pin can cause a voltage drop. Connector terminal fretting or corrosion is also common.

4. Poor ground connection at the relay or ECM

A loose or corroded ground for the relay coil or the ECM itself can affect the sense circuit reading, making it appear low.

5. Faulty ECM/PCM (rare)

Internal damage to the ECM’s sense circuit input is possible but far less likely than external circuit issues. This should only be considered after all other causes are ruled out.

How it's diagnosed

Check the ECM/PCM power relay by swapping it with an identical relay from a non-critical circuit (e.g., horn) and see if the code returns or the symptom changes.
Inspect the ECM/PCM power relay fuse for continuity and clean, tight connections. Replace if blown and investigate the cause of the overload.
Visually examine the wiring harness and connectors between the relay and the ECM for signs of damage, corrosion, or loose pins. Pay special attention to the sense wire (often a smaller gauge wire from the relay output to the ECM).
With the key on, measure voltage at the relay output terminal and at the ECM sense pin. Compare to battery voltage; a drop of more than 0.5V indicates high resistance in the circuit.
Check the relay ground and ECM ground connections for cleanliness and tightness. Perform a voltage drop test on the ground side while the circuit is under load.
If all external circuits test good, verify the ECM’s ability to read the sense voltage by back-probing the sense pin and using a scan tool to compare the reported value. Only then suspect an internal ECM fault.

Code facts & related codes

SystemPowertrain (engine & transmission)

StandardizationSAE generic — identical on every make

Frequently asked

Can a bad ECM/PCM cause a P0689 code?

Yes, but it is very rare. The ECM monitors the sense circuit, so an internal fault could misinterpret voltage. However, in most cases, the problem is a faulty relay, blown fuse, or wiring issue. Replacing the ECM should be a last resort after thorough circuit testing.

I was told I need a new PCM for P0689. Is that usually correct?

No, it is a common misdiagnosis. Many shops or parts stores may suggest a PCM replacement because the code mentions the ECM/PCM, but the actual fault is almost always in the power relay, its fuse, or the wiring. Replacing the PCM without fixing the underlying circuit issue will not solve the problem and is an unnecessary expense.

Can a weak battery cause this code?

A weak battery alone is unlikely to cause P0689, because the sense circuit is designed to monitor the relay output relative to system voltage. However, if the battery is so low that the relay cannot close properly, it might contribute. Always ensure the battery is fully charged and connections are clean before diagnosing.

Will the car start with a P0689 code?

It depends on the severity. If the relay is intermittently failing or the sense circuit has a partial connection, the engine may start and run. If the relay is completely open or the fuse is blown, the ECM will not receive power and the engine will not start.