P25AF: Coolant Temperature Control Module Requested MIL Illumination

What it means

A dedicated coolant-temperature control module (used on engines with electric coolant pumps and electronic thermostats to keep the cylinder head and block at independent setpoints) has requested the ECM to illuminate the MIL because of an emissions-relevant fault on its side. Original fault is in that module.

At a glance

• Usually lights the check-engine light

How it works

On vehicles with a separate coolant-temperature control module, that module manages electric coolant pumps or electronic thermostats to regulate engine temperatures. P25AF means it has asked the engine control module to turn on the check-engine light because it detected an emissions-relevant fault; the originating fault is usually stored in the coolant module.

Common causes

Most likely first.

1. Internal coolant temperature control module fault
2. Module power, ground, or wiring fault
3. Communication fault between the coolant module and engine control module

Symptoms

• Check-engine light on
• Coolant-temperature or engine-overheating warning may appear
• Cooling fan may run unusually or engine temperature may fluctuate

How to diagnose it

1. Scan both the engine control module and coolant-temperature control module, recording their codes and freeze-frame data before clearing them.
2. Check live coolant-temperature readings for implausible values or fluctuations and compare them with the engine’s actual temperature.
3. Inspect the coolant module’s power, ground, connectors, and wiring for loose connections, corrosion, or damage.
4. Check communication status between the coolant module and engine control module and look for related network codes.
5. Follow the coolant module’s own fault information to test the affected pump, thermostat, or internal module function against the manufacturer's procedure.

Common fixes

• Repair a loose, corroded, or damaged module connector or wire.
• Restore the coolant module’s power or ground connection if faulty.
• Repair communication wiring if the modules are not exchanging messages reliably.
• Replace a failed electric coolant pump or electronic thermostat identified by the coolant module’s diagnostic information.
• Replace or reprogram the coolant-temperature control module only if its own tests identify an internal fault.

Can I keep driving?

Watch the temperature gauge and heed any coolant-temperature or overheating warning; stop safely if the engine overheats. Avoid long or high-load trips until the cooling-module fault is identified, since coolant flow or temperature control may be unreliable.