Powertrain, generic · Computer and auxiliary outputs

P0686 ECM/PCM Power Relay Control Circuit Low

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. P0686 means the control line is shorted to ground: with the driver off it should carry battery voltage through the coil, but it sits at ground (0.5 Ω or less). The relay then stays energised and the module can no longer switch it off – after-run and shutdown are disturbed and the battery can drain. 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 (the wording of P0685) was a shifted entry; P0686 is the 'low' code.

Likely causes

Control wire between relay and module chafed to ground
Relay shorted internally or water in the relay carrier
Module driver shorted internally

Symptoms

  • Check engine light
  • Relay stays on after ignition off, battery drains
  • Possibly P068B as well

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

P0685P0687P068BP0690P0689

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