Powertrain, generic · Computer and auxiliary outputs

P0685 ECM/PCM Power Relay Control Circuit/Open

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. P0685 means the relay control circuit is open: GM measures with the driver off and flags 200 kΩ or more to ground. 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. GM Europe's 2.0 L turbo compares commanded and actual state for 2 s, Ford's 2023 diesel checks continuously within 0.5 s. A real open keeps the relay from pulling in – no start; on Saab's V6 there is then no communication with the engine module and the MIL flashes. GM notes P0686 may set alongside.

Likely causes

Open or corroded control wire between relay coil and module or connector
Relay coil open or relay badly seated
Relay coil supply missing (fuse, battery positive)
Weak battery or voltage dip
Module driver faulty

Symptoms

  • Check engine light
  • Engine does not start or stalls
  • No communication with the engine module (Saab)
  • MIL flashes rapidly, other relays click (Saab)

Repair estimate

DifficultyMediumDIY possible
Parts and labour€20–350
Time0.5–2.5 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

P0686P0687P0688P0689P0690P068A

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