EngineLightCodes
Powertrain code

P0687 ECM/PCM Power Relay Control Circuit High

Severity
Fix soon
Typical repair cost
$20–$390
Do it yourself
Possible, medium
Shop time
0.5–3 hours

The engine module switches its main relay coil to ground (control circuit), reads the switched voltage back on a sense line and keeps the relay on briefly after ignition off to save learned values.

How the engine computer detects it

P0687 means the control line is shorted to voltage: when the driver switches on and should pull it to ground, it stays high (0.5 Ω or less to positive) or the driver goes into overcurrent protection, so the relay does not pull in – no supply to the loads, no start or stalling. GM checks with run/crank voltage above 11 V and sets the code after 8 of 10 samples at 250 ms, with the lamp after two trips. The former obdex title (a sense-circuit wording) was a shifted entry; P0687 is the 'high' code of the control circuit. A high voltage on the sense line is P0690.

Written from public technical sources and not yet checked by a mechanic. How we write these pages

Symptoms

  • Check engine light
  • Engine does not start or stalls
  • Relay does not click

Common causes

  1. Control wire shorted to positive (harness, external voltage) Most likely
  2. Relay coil shorted (very low resistance) Possible
  3. Module driver faulty Less likely

How to diagnose P0687

  1. Read every stored code with an OBD-II scanner. If misfire or fuel trim codes are there too, fix those first: they often cause this one.
  2. Inspect the parts involved: ecm power relay, relay control circuit, engine control module.
  3. Work through the causes above from most to least likely, cheapest checks first.
  4. Clear the code and drive a full drive cycle to confirm the repair held.

Reading the code

P
Powertrain
0
Generic (SAE) code
6
Computer and output circuits
87
Specific fault

Questions drivers ask

Can I drive with code P0687?

Usually yes for a short time, if the car drives normally, the temperature gauge reads normal and the check engine light is steady. If the light flashes, the engine loses power or overheats, or other warning lights come on, pull over and have it checked.

Will my car pass an emissions inspection with P0687?

Probably not in states that test vehicles with an OBD-II scan: an emissions-related code that keeps the check engine light on is usually an automatic fail. Rules differ by state and county, so check yours.

Will P0687 clear by itself?

The light can go off after several drive cycles without the fault, but the code returns if the cause is still there. After a repair, the readiness monitors need a full drive cycle before an inspection.

Can I fix P0687 myself?

Often yes, with an OBD-II scanner and basic tools. Difficulty: medium. Start with the cheapest checks before replacing expensive parts.

This page is general information, not a substitute for a professional diagnosis. Repair costs are rough estimates for parts and labor and vary by vehicle and region.