Powertrain, generic · Computer and auxiliary outputs

P0687 ECM/PCM Power Relay Control Circuit High

Check engine lightOn
EmissionsRelevant
Drive cycleRequired
Limp modeNo

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. 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.

Likely causes

Control wire shorted to positive (harness, external voltage)
Relay coil shorted (very low resistance)
Module driver faulty

Symptoms

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

Repair estimate

DifficultyMediumDIY possible
Parts and labour€20–350
Time0.5–3 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

ecm power relayrelay control circuitengine control module

Related codes

P0685P0686P0690P0689P0688

References: law.cornell.edu, gsi.ext.gm.com, gsi.ext.gm.com, saabwisonline.com.