P0646 code meaning
A/C Clutch Relay Control Circuit Low
What P0646 means
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.
P0646 at a glance
- Check engine lightUsually not
- Emissions testNot an emissions code
- Limp modeNot expected
- Repair difficultyEasy, DIY possible
- Typical labor0.3 to 1.5 hours
- Clears after repairWhen cleared with a scanner
Symptoms of P0646
- A/C compressor runs constantly, even with A/C off
- Evaporator ices up, cooling drops
- With an open circuit: no cooling
Common causes of P0646
- Most likelyControl wire between relay and module shorted to ground (chafing)
- PossibleRelay faulty internally (coil, ground)
- PossibleMoisture or corrosion in the connector
- Less likelyOutput driver in the module faulty
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Ac clutch relayRelay control circuitEngine control module
What each part of P0646 means
| P | Powertrain system: engine, fuel system, ignition, emissions and transmission. |
| 0 | Generic code: it means the same on every make (SAE standard). |
| 6 | Area: computer (engine control module) and output circuits. |
| 46 | The exact fault in that area. |
What to do next
- Note P0646 and any other codes stored with it: several codes together often point to one cause.
- Check for loose connectors and damaged wiring around the parts the code names.
- Check open recalls for your car: some faults are fixed for free by the dealer. Enter your VIN.
- Clear the code only after the repair. If it comes back, the fault is still there.
Buying a used car? Scan it for stored codes before you pay. A recently cleared memory can hide a problem.
Questions about P0646
What does P0646 mean?
P0646 means "A/C Clutch Relay Control Circuit Low". The engine module switches the A/C compressor clutch relay through a low-side driver once the climate control requests cooling. It is a generic code, so it means the same on every make.
Is it safe to drive with P0646?
P0646 does not normally turn on the check engine light by itself. Watch for the symptoms above and have it checked soon.
Will P0646 fail an emissions test?
P0646 is not an emissions code, so on its own it usually does not fail an emissions test. Other stored codes or a lit check engine light still can.
How hard is P0646 to fix?
Difficulty: easy. A typical repair takes 0.3 to 1.5 hours of work. A careful DIY repair is possible with basic tools and a scanner.
Will P0646 clear itself after the repair?
Clear the code with a scanner after the repair. If the fault is fixed, it does not come back.