P0646 means A/C Clutch Relay Control Circuit Low. The A/C clutch relay control circuit voltage is lower than expected, often from a short to ground. It is a generic OBD-II code, so the definition is the same on any make, but the most likely cause varies by engine; confirm with a scan tool and the service information for your vehicle before buying parts.
What P0646 means
Modern vehicles have many control modules that share information over a network, usually called the CAN bus. Codes starting with U mean a module has stopped hearing another one, and voltage and processor codes mean a module is not getting clean power or has an internal fault. Low battery voltage, poor grounds, blown fuses and damaged wiring cause many of these codes, so test the battery and charging system first.
P0646 symptoms
- A/C not cooling
- Compressor clutch not engaging or staying on
- Check engine light
Common causes of P0646
- Failed A/C clutch relay
- Wiring or connector fault
- Engine computer driver fault
How to diagnose P0646
- Swap or test the A/C clutch relay
- Check the relay control circuit for shorts or opens
- Check the fuse feeding the relay
Can you drive with P0646?
Yes; the A/C may not work.
P0646 tips
Check the relay socket for melted or corroded terminals, which can short the control circuit.
Will P0646 clear itself?
If the fault stops, the computer turns off the check engine light by itself after several drive cycles without seeing it again, and the code stays in memory as history for a while. Clearing P0646 with a scan tool after a repair is fine, but if the cause is still there the code returns, sometimes within one drive. Disconnecting the battery also clears codes on most vehicles, but it erases learned fuel trims and idle settings as well.
Related codes
| Code | Meaning |
|---|---|
| P0645 | A/C Clutch Relay Control Circuit |
| P0647 | A/C Clutch Relay Control Circuit High |
| P0532 | A/C Refrigerant Pressure Sensor A Circuit Low |
Guides that help
When it is more than a sensor
Voltage and network codes come from wiring, the battery, the charging system or a module, not from engine or transmission wear. If a lost-communication code has left the transmission in limp mode, rule out the module and wiring before deciding the unit has failed, and see our transmissions only if testing confirms internal damage.
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