P0646A/C Clutch Relay Control Circuit Low
- Generic OBD-II code · Powertrain
- How the car detects it
- The engine module switches the A/C compressor clutch relay through a low-side driver once the climate control requests cooling. P0646 means the voltage at the control output is too low while the driver is off – the control wire is shorted to ground, so the relay can pull in permanently and the compressor keeps running, which can ice up the evaporator. Saab's EDC16 diesels set it after more than 0.6 s with the engine running; GM Europe's 2011 Saab 9-5 also includes an open circuit here. Ford's OBD summaries list P0646/P0647 in a 5 V reference section for Bosch modules – an unusual assignment not covered here. The code is not emission relevant and does not light the check engine lamp at most makers.
- What you may notice
- A/C compressor runs constantly, even with A/C off
- Evaporator ices up, cooling drops
- With an open circuit: no cooling
- Likely causes
- LikelyControl wire between relay and module shorted to ground (chafing)
- PossibleRelay faulty internally (coil, ground)
- PossibleMoisture or corrosion in the connector
- RareOutput driver in the module faulty
- Repair
- Easy · Often possible to do yourself.Typical cost: €15 – €250
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.