P0688 Code: Meaning, Causes, and Symptoms | OBD2 Codes
P0688 – Electronic Control Module Power Relay Sense Circuit/Open
P0688 Possible Symptoms
Engine Light ON (or Service Engine Soon Warning Light)
P0688 Possible Causes
Weak battery,Low voltage while cranking,Faulty Engine Control Module (ECM) power relay,ECM power relay harness is open or shorted,ECM power relay circuit poor electrical connection,Blown ECM fuse,Faulty ECM
The powertrain relay is a normally open relay. The relay armature is held in the open position by spring tension. Battery positive voltage is supplied directly to the relay coil and the armature contact at all times. The Engine Control Module (ECM) supplies the ground path to the relay coil control circuit via an internal integrated circuit called an Output Driver Module (ODM). The ODM output control is configured to operate as a low side driver for the powertrain relay. The ODM for the powertrain relay also incorporates a fault detection circuit, which is continuously monitored by the ECM. When the ECM commands the powertrain relay ON, ignition 1 voltage is supplied to the ECM, and to several additional circuits.