Fault inside the compressor control module, such as the driver temperature sensor or its internal circuit; Damaged wiring or a short to ground in the sensor circuit, where the sensor is external or wired separately (model dependent); Corroded, wet or loose connector terminals at the control module or compressor; Poor module power supply or earth connection causing implausible readings; Water ingress or physical damage to the compressor or module housing.

P0D78

A/C Compressor Control Module Output Driver Temperature Sensor Circuit Low.

Generic
Powertrain
Generic code
Medium severity

How this code is built

P: SystemPowertrain
0: Defined bySAE, the same on every car
D: AreaReserved
78: FaultThe specific fault in that area

What it means

P0D78 means the A/C compressor control module has seen the signal from its output driver temperature sensor circuit stay lower than expected. The module monitors the temperature of its own output driver stage to protect it from overheating. A low-voltage or short-to-ground reading is outside the plausible range, so the code is set. Where the sensor sits and how the module is built depends on the model, and it is most often found on vehicles with a separate electric compressor control module.

Symptoms

A/C blowing warm or not cooling
A/C compressor not engaging or cutting out
Warning light on, with the fault stored in the A/C or compressor control module
Reduced or disabled A/C operation as a protective measure
On hybrid or electric models, possible reduced cooling of the battery or related systems (model dependent)

Possible causes

Fault inside the compressor control module, such as the driver temperature sensor or its internal circuitlikely
Damaged wiring or a short to ground in the sensor circuit, where the sensor is external or wired separately (model dependent)possible
Corroded, wet or loose connector terminals at the control module or compressorpossible
Poor module power supply or earth connection causing implausible readingspossible
Water ingress or physical damage to the compressor or module housingrare

How to diagnose

Read all stored codes and freeze frame data, and note whether other A/C, power supply or communication codes are present
Check the fault status: clear it once and see if it returns straight away (constant fault) or only under certain conditions
Inspect the compressor and control module wiring and connectors for chafing, corrosion, water ingress, heat damage or loose pins
Check the battery condition, relevant fuses and the module's power and earth connections against the manufacturer's wiring diagram
Check live data for the driver temperature reading and compare it with ambient and compressor temperature; a reading stuck at an extreme suggests a sensor or module fault
Where the sensor circuit is accessible, test it for short to ground and measure resistance or voltage to the manufacturer's specification
Check whether the manufacturer has published a software update or service procedure for this code on the specific model
Confirm the module is at fault only after wiring, supply and connections have been proven good

How to fix

Repair or replace damaged wiring and ensure the circuit is free of shorts to ground
Clean, repair or replace corroded or loose connectors and terminals
Restore good power and earth connections to the control module
Replace the compressor control module or compressor assembly if the internal sensor or driver has failed, following manufacturer procedures
Dry out and repair any water ingress source, then replace affected components
Apply any manufacturer-issued software update only if it is a known fix for this fault

Can I keep driving?

Medium severity

The car can usually be driven, but the A/C may be disabled, so book a repair soon. If other warnings appear or the vehicle is a hybrid or electric model showing reduced power or cooling warnings, limit driving and have it checked promptly.

Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.