P0687🔧 Engine computer power supply (PCM power relay)Stop and fix promptly

P0687: ECM/PCM Power Relay Control Circuit High (Meaning, Causes, Fixes)

Reviewed 2026-10-25 · Based on 2 cited source(s)

What P0687 means

Most OBD-II vehicles feed battery voltage to the powertrain computer (PCM) through a relay, though OBD-Codes.com notes a few makers still use only a fused circuit. When the ignition is switched on, a small coil in the relay is energized (often with a light click), its contacts close, and battery voltage flows to the PCM and, on many designs, to a controller area network (CAN) signal terminal. OBD-Codes.com says P0687 is stored when a high voltage condition is detected, by the PCM or another controller, in the circuit that powers the PCM or the circuit other controllers use to monitor PCM supply voltage. RepairPal puts it more broadly: the PCM sets P0687 if it detects an incorrect voltage signal from the power relay. It is a generic code for 1996 and newer vehicles, but causes can differ a little between brands. The sources disagree on symptoms. OBD-Codes.com says P0687 is not usually accompanied by a no-start (though some vehicles disable the PCM), while RepairPal lists 'engine may not start' and 'hard or no start in cold weather'. They also differ on causes: OBD-Codes.com puts an overcharging alternator first and adds the ignition switch and shorted wiring, while RepairPal lists PCM failure, power relay failure and wiring. P0685 (circuit open) and P0686 (circuit low) cover the same relay circuit.

Severity: can I keep driving?

Stop and fix promptly

Get it fixed soon. RepairPal rates P0687 Urgent, meaning immediate action is needed. OBD-Codes.com says the car usually still starts, though some vehicles shut the engine computer down when this code is stored, and warns that the extra voltage can damage the engine computer or other control modules even if the car seems to drive fine.

Common symptoms

Likely causes

Roughly most to least common, based on the sources below. Your vehicle may differ.

  1. Alternator overcharging (OBD-Codes.com)
  2. Bad PCM power relay (OBD-Codes.com, RepairPal)
  3. Faulty ignition switch (OBD-Codes.com)
  4. Shorted wiring or connectors (OBD-Codes.com); wiring issue (RepairPal)
  5. Faulty engine computer (PCM) (RepairPal)

Diagnostic steps you can do

⚠ Before you start

Work on a cool engine, parked on level ground with the parking brake set. Never work under a car supported only by a jack.

  1. Check for technical service bulletins (TSBs) for your vehicle first; OBD-Codes.com notes your problem may be a known issue with a known fix.
  2. Test the charging system. OBD-Codes.com says to check that the alternator is not overcharging before anything else, and to diagnose any charging light or overcharging code before P0687.
  3. Look over the wiring, connectors, battery and battery cable ends for looseness and corrosion.
  4. Turn the key on and feel or listen for the PCM relay to click. OBD-Codes.com says to swap only identical relays (matching part numbers) for testing, and put relays back in their original spots.
  5. With a multimeter and a wiring diagram, OBD-Codes.com describes checking, with the engine running, for excessive voltage at the relay's primary input (points to the alternator) and secondary input (points to a short to voltage, especially at the ignition switch), and for voltage on the relay's ground terminal. Disconnect related computers before measuring resistance.

Typical fixes

Repair costs vary widely by vehicle, region, parts brand and shop, so we don't quote prices. Get a written estimate after diagnosis.

🛠 When to call a pro

  • The charging system reads high and you can't test the alternator yourself
  • You don't have a multimeter or a wiring diagram for the PCM power circuit
  • The car won't start or other control modules are acting up
  • Tests point to the engine computer, which may need programming to your vehicle

Where to buy

Plain search links. Always match parts to your exact year, make, model and engine.

OBD2 scanners on Amazon Parts at Advance Auto Parts catalog at RockAuto

FAQs

Can I drive with P0687?

Often the car still runs, but OBD-Codes.com says excessive voltage can damage the engine computer or other modules, and RepairPal rates P0687 Urgent. Get it checked soon, starting with the alternator.

What's the difference between P0685, P0686 and P0687?

All three involve the relay that powers the engine computer. P0685 is circuit open, P0686 is circuit low (low or no voltage), and P0687 is circuit high (too much voltage). OBD-Codes.com puts an overcharging alternator at the top of the P0687 cause list.

Will P0687 stop my car from starting?

The sources disagree. OBD-Codes.com says it usually doesn't, though some vehicles disable the engine computer when it's stored. RepairPal lists 'engine may not start' and a hard or no start in cold weather.

How much does it cost to fix P0687?

Neither source gives a repair cost for P0687. It can be an alternator, a relay, an ignition switch or wiring, so diagnose it before buying parts.

Sources

  1. OBD-Codes.com: P0687 ECM/PCM Power Relay Control Circuit High
  2. RepairPal: P0687 ECM/PCM Power Relay Control Circuit High (rated Urgent)

Last reviewed 2026-10-25. Code definitions and repair steps can vary by manufacturer. Confirm with your vehicle's service information.