What does it mean?
The module has measured a voltage on the charge air cooler cooling fan control circuit that is lower than the state it commanded should produce. When the driver is switched off the line should sit near supply voltage; a line held near ground instead means current is finding a path it should not have. This is a straightforward electrical fault and the module will normally disable the output to protect itself, which leaves the fan unavailable. It is worth remembering that a motor which has shorted internally looks exactly like a shorted wire from the module's point of view, so the load has to be separated from the harness during the test. Once the output is disabled, intake temperatures rise in slow traffic and power is limited.
Symptoms
- MIL on, often with a blown fuse in the fan circuit
- Fan does not run at all, even when the module tries to command it
- Intake air temperature rising in traffic and staying high
- Power reduction and boost limiting under load, particularly in warm weather
- Repeated fuse failures if the short is still present after a fuse replacement
Likely causes
- Control wire shorted to ground where the harness runs across the front structure or a bracket edge
- Internally shorted fan motor or fan control module pulling the circuit down
- Water and salt inside the fan connector creating a low resistance path
- Crushed or pinched wiring after a front end repair or a minor impact
- Corroded terminal bridging to the connector shell or to a neighbouring ground pin
- Relay coil shorted internally on relay driven systems
- Failed output driver inside the control module, accepted only when the harness and the fan test good
How to fix it
- Check the fuse for that circuit; a blown fuse immediately supports a short to ground
- Disconnect the fan and see whether the low voltage condition disappears, which separates the motor from the wiring
- With the fan unplugged, measure the control wire to ground along its whole length
- Measure the fan motor windings for a short to its own housing
- Inspect the connector for water, corrosion and terminals touching the shell
- Follow the harness across the front of the vehicle looking for chafe and crush points, and check any recent repair area
- Repair the harness or replace the fan when it is proven internally shorted, then command the fan and confirm the circuit voltage now matches the commanded state