P0688
UniversalThe Powertrain Control Module (PCM) or Engine Control Module (ECM) has detected an unexpected voltage reading on the power relay sense circuit, indicating a potential issue with the relay's operation or its associated wiring.
Medium SeverityThe Powertrain Control Module (PCM) or Engine Control Module (ECM) is not receiving the correct voltage signal from its power relay, which provides power to the computer. This can lead to various drivability issues, including a no-start condition or the engine stalling while driving.
⚠️ Before You Begin (Safety First)
Always disconnect the battery before working on electrical components to prevent electrical shock or damage to the vehicle's electrical system. Allow hot engine components to cool down before touching them.
Common Symptoms
- Check Engine Light illuminated
- Engine cranks but does not start
- Engine stalls unexpectedly while driving
- Intermittent starting issues
- Various warning lights on the dashboard
Common Causes
- Faulty ECM/PCM power relay
- Corroded or loose wiring/connections in the power relay circuit
- Short circuit or open circuit in the power relay wiring
- Faulty fuse for the ECM/PCM power supply
- Damaged ECM/PCM (less common but possible)
Frequently reported on: General/Universal
Diagnostic Steps
- Check for blown fuses related to the ECM/PCM power supply.
- Locate the ECM/PCM power relay (refer to your vehicle's service manual or fuse box diagram) and inspect it for signs of corrosion or damage.
- With the battery disconnected, swap the ECM/PCM power relay with a known good, identical relay from another non-critical system (e.g., horn relay if it's the same type) to test if the relay is faulty.
- Inspect wiring and connectors leading to and from the power relay for any visible damage, chafing, or corrosion.
- Clear the code with an OBD-II scanner after performing repairs and test drive the vehicle to ensure the issue is resolved.
👨🔧 When to Call a Professional
If the vehicle repeatedly stalls while driving or refuses to start after replacing the power relay, seek professional diagnosis immediately. This could indicate a deeper wiring harness issue or internal PCM fault.
Commonly Asked Questions
What is the ECM/PCM power relay?
It's an electrical switch controlled by the vehicle's computer that supplies power to the Engine Control Module (ECM) or Powertrain Control Module (PCM), which are essential for engine operation.
Can I drive with a P0688 code?
It is not recommended to drive with a P0688 code, as it can cause the engine to stall unexpectedly or prevent it from starting, leading to dangerous situations.
Where is the ECM/PCM power relay located?
Its location varies by vehicle, but it's typically found in the under-hood fuse box or an auxiliary fuse/relay box. Consult your vehicle's owner's manual or service diagram.
Is this an expensive fix?
Replacing the power relay itself is usually inexpensive. However, if the issue is a complex wiring problem or a faulty PCM, the repair costs can increase significantly.
Why is my car cranking but not starting with this code?
The P0688 code suggests the PCM is not receiving proper power. If the PCM doesn't get power, it cannot control essential engine functions like fuel delivery and ignition, preventing the engine from starting.