P0646
A/C Clutch Relay Control Circuit Low
- Severity
- Low
- Driving
- No driving restriction
The air conditioning clutch relay circuit is reading low - typically shorted to earth or an open circuit. The module cannot control the compressor properly, so it may never engage or may stay engaged when the engine needs the power.
Possible causes
- CommonFailed relay
- CommonCorroded relay socket or fuse box connections
- CommonWiring fault between the module and the relay
- CommonWater ingress into the fuse box
- PossibleBlown fuse
- RareFailed control module driver
What you might notice
- Air conditioning not working
- Compressor staying engaged
- Noticeable loss of power with the air conditioning on
- Check engine light on
What to check, in order
- Check whether the low reading persists with the relay removed.
- Swap the relay with an identical one from elsewhere in the box and see whether the fault follows it. It costs nothing and is the fastest test available.
- Inspect the relay socket for corrosion, heat damage and spread terminals - a discoloured socket means the relay has been running hot.
- Check the fuse for the circuit.
- Check the fuse box for water ingress, particularly if it is mounted low or in the engine bay.
- Check the wiring between the module and the relay.
Read this code from your own car
Plug an ELM327 adapter into the OBD-II port under the dashboard. Pin16 reads every stored, pending and permanent code, explains each one like this page does — offline, with no account — and shows what the car recorded when the fault was set.
Where this code sits
P00xxP01xxP02xxP03xxP04xxP05xxP06xxP07xxP08xxP09xxP0AxxP0BxxP0CxxP0DxxP0ExxP0FxxP10xxP11xxP12xxP13xxP14xxP15xxP16xxP17xxP18xxP19xxP1AxxP1BxxP1CxxP1DxxP1ExxP1FxxP20xxP21xxP22xxP23xxP24xxP25xxP26xxP27xxP28xxP29xxP2AxxP2BxxP2CxxP2DxxP2ExxP2FxxP30xxP31xxP32xxP33xxP34xxP35xxP36xxP37xxP38xxP39xxP3AxxP3BxxP3CxxP3DxxP3ExxP3Fxx
Computer output circuits and control module
P0600P0601P0602P0603P0604P0605P0606P0607P0608P0609P0610P0611P0612P0613P0614P0615P0616P0617P0618P0619P0620P0621P0622P0623P0624P0625P0626P0627P0628P0629P0630P0631P0632P0633P0634P0635P0636P0637P0638P0639P0640P0641P0642P0643P0644P0645P0646P0647P0648P0649P0650P0651P0652P0653P0654P0655P0656P0657P0658P0659P0660P0661P0662P0663P0664P0665P0666P0667P0668P0669P0670P0671P0672P0673P0674P0675P0676P0677P0678P0679P0680P0681P0682P0683P0684P0685P0686P0687P0688P0689P0690P0691P0692P0693P0694P0695P0696P0697P0698P0699