Faulty or sticking ECM/PCM main power relay; Open circuit, high resistance or corrosion in the relay sense wiring or relay output feed; Poor connection at the relay socket, fuse box terminals or control module connector; Blown or loose fuse feeding the relay or the control module power supply; Weak battery, poor battery terminals or a charging system fault causing low supply voltage; Poor earth connection for the relay or control module; Internal fault in the control module.

P0688

ECM/PCM Power Relay Sense Circuit/Open.

Generic
Powertrain
Generic code
High severity

How this code is built

P: SystemPowertrain
0: Defined bySAE, the same on every car
6: AreaComputer and output circuits
88: FaultThe specific fault in that area

What it means

P0688 means the engine or powertrain control module has detected that the circuit monitoring the main power relay output is open, or that the voltage there is lower than expected. The control module checks this sense line after the relay is commanded to switch off, and when it is on. If the sense voltage does not follow the relay state, the fault is set. The exact relay layout and wiring vary by vehicle, so check the model-specific wiring diagram.

Symptoms

Engine cranks but fails to start, or starts intermittently
Engine stalls unexpectedly while driving or idling
Malfunction indicator lamp illuminated
Hesitation or loss of power
Other unrelated fault codes stored, or a loss of communication with the control module
Control module stays awake after switch-off, leading to battery drain

Possible causes

Faulty or sticking ECM/PCM main power relaylikely
Open circuit, high resistance or corrosion in the relay sense wiring or relay output feedlikely
Poor connection at the relay socket, fuse box terminals or control module connectorlikely
Blown or loose fuse feeding the relay or the control module power supplypossible
Weak battery, poor battery terminals or a charging system fault causing low supply voltagepossible
Poor earth connection for the relay or control modulepossible
Internal fault in the control modulerare

How to diagnose

Read all stored codes and the freeze frame data, noting battery voltage, engine speed and whether the fault is current or intermittent
Check the battery state of charge and terminal condition, and test charging system output
Inspect the relevant fuses and the main relay for damage, heat marks, corrosion or a loose fit
Inspect the relay socket, fuse box and control module connectors for water ingress, bent or pushed-back pins and green corrosion
With the wiring diagram for the model, check the relay coil feed, switched output and sense circuit for continuity and voltage drop, and wiggle test the harness while monitoring
Swap in a known good relay of the identical type, or bench test the relay coil and contacts, if the diagram confirms it is suitable
Check the control module power and earth connections under load, with ignition on and while cranking
Only after wiring and relay are proven good, consider testing or replacing the control module, following the manufacturer's procedure

How to fix

Replace the faulty main power relay with the correct specification part
Repair or replace damaged wiring, and clean or renew corroded connectors and terminals in the sense or supply circuit
Replace blown fuses after finding and correcting the cause of the overload
Clean and tighten battery terminals and earth points, and replace the battery or repair the charging system if at fault
Repair or replace the control module only if confirmed faulty, and carry out any programming the manufacturer requires

Can I keep driving?

High severity

Avoid driving until it is diagnosed, as the engine may stall without warning, which is unsafe in traffic. If the car is running normally and the fault is intermittent, keep trips short and get it checked as soon as possible.

Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.