P0647

What it means

P0647 means the engine control module has detected a high voltage on the A/C clutch relay control circuit. The module drives this circuit to switch the relay that engages the compressor clutch, and it saw a voltage higher than expected for the commanded state, such as high when it was commanding the relay off. The exact way the circuit is monitored depends on the vehicle.

Symptoms

• A/C blows warm or does not cool
• A/C compressor clutch does not engage or engages constantly
• Compressor clutch stuck on with A/C switched off
• Warning light on, possibly with A/C indicator flashing or inoperative
• Slightly higher idle or poor fuel economy if the compressor runs continuously

Possible causes

• Faulty A/C clutch relay (internal short or stuck contacts, or failed coil/driver diode)likely
• Control circuit wiring shorted to voltage, or chafed insulation touching a live feedlikely
• Corroded or damaged connector terminals or relay base in the fuse boxpossible
• Open circuit or high resistance in the relay control or earth path causing a false high readingpossible
• Fault in the engine control module relay driver outputrare
• Aftermarket wiring or accessories interfering with the A/C circuitrare

How to diagnose

1. Read the stored codes and freeze frame data, noting whether A/C was requested and any other related codes
2. Check the A/C clutch relay and its fuse, and inspect the relay base and terminals for heat damage, corrosion or water ingress
3. Swap the relay with a known good identical one and retest, if the wiring diagram confirms it is the same type
4. Inspect the control circuit wiring and connectors between the relay and the control module for chafing, melting or pinning faults
5. With the ignition on and the relay removed, measure the control circuit voltage at the relay socket against the wiring diagram, with A/C on and off
6. Check the relay control circuit for a short to voltage and for continuity and resistance in the earth path, with the module disconnected
7. Check the compressor clutch coil resistance and its feed wiring against the manufacturer's specification
8. If all wiring and the relay are good, test the module output and check for software updates, as procedures vary by model

How to fix

• Replace the faulty A/C clutch relay with the correct specification part
• Repair or replace shorted, chafed or damaged wiring and secure it away from live feeds
• Clean or replace corroded connectors, terminals or the relay base
• Restore the open circuit or poor earth connection found during testing
• Replace or repair the engine control module only after confirming wiring and relay are sound
• Remove any faulty aftermarket wiring affecting the circuit, then clear the code and verify A/C operation

Can I keep driving?

Nearby codes

• P0646GenericA/C Clutch Relay Control Circuit Low
• P0648GenericImmobilizer Lamp Control Circuit