P0687 indicates that the engine control module (ECM/PCM) sees the voltage on its power‑relay control circuit higher than expected when the relay should be off. In practice, this means the control line is stuck high, suggesting a short to battery voltage somewhere in the relay coil or its wiring rather than a failure of the computer itself.
The relay’s coil can develop an internal short, causing the control circuit to remain powered even when the ECM commands it off.
A damaged harness—often from chafing, heat damage, or pinched wires—can create a direct short to battery voltage on the signal line.
Moisture intrusion or connector pins that are bent can allow voltage to leak onto the control line, producing a high reading.
If an alternate power source is feeding the relay coil through a faulty switch, the line will stay high regardless of ECM commands.
A poor ground can cause the control circuit to see stray voltage, though this usually results in a low rather than high condition; it is listed for completeness.
The ECM itself can fail to properly toggle the relay driver, but this is the least common cause and should be confirmed only after all other possibilities are ruled out.
No. In most cases the fault lies in the relay coil or the wiring that feeds it. The ECM is rarely at fault and should be replaced only after all other components have been tested and proven good.
A low battery may cause starting problems, but it does not create a high‑voltage condition on the relay control line. P0687 points to a short to power, not a voltage‑drop issue.
The vehicle may stall or fail to start, and the relay may stay energized, draining the battery. It is advisable to diagnose and repair the issue before extended driving.
A clicking relay indicates the coil is being mechanically activated, but an electrical short can keep the control circuit high. Testing the circuit’s voltage with a multimeter while the relay is commanded off will reveal whether the line is still powered.