Powertrain, generic · Computer and auxiliary outputs

P0646 A/C Clutch Relay Control Circuit Low

Check engine lightUsually off
EmissionsNot relevant
Drive cycleNot required
Limp modeNo

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.

Likely causes

Control wire between relay and module shorted to ground (chafing)
Relay faulty internally (coil, ground)
Moisture or corrosion in the connector
Output driver in the module faulty

Symptoms

  • A/C compressor runs constantly, even with A/C off
  • Evaporator ices up, cooling drops
  • With an open circuit: no cooling

Repair estimate

DifficultyEasyDIY possible
Parts and labour€15–250
Time0.3–1.5 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

ac clutch relayrelay control circuitengine control module

Related codes

P0645P0647P0532P0533

References: law.cornell.edu, saabwisonline.com, saabwisonline.com.