P2604 indicates a range/performance problem in the intake air heater circuit, commonly found on diesel engines. The heater (grid heater) is not operating within its expected parameters, which can affect cold-start performance and may trigger the check engine light.
The heater grid itself can burn out, crack, or develop high resistance, causing the circuit to fall outside expected range.
A stuck or failed relay can prevent the heater from turning on or off correctly, leading to a range/performance fault.
Corroded, loose, or damaged wires/connectors in the heater circuit can cause intermittent or incorrect resistance readings.
If the sensor provides inaccurate temperature data, the PCM may command the heater incorrectly, triggering the code.
Rare; a failed control module can misread or fail to drive the heater circuit, but this is the least likely cause and should only be considered after all other components are verified.
You may be able to drive, especially in warm weather, but hard starting in cold conditions and potential battery drain if the heater stays on are risks. It's best to diagnose and repair promptly.
Yes, if you have basic mechanical skills and follow safety precautions (disconnect battery, avoid contact with high-current circuits). However, ensure you have the correct replacement part and torque specifications.
Almost never. The PCM is rarely the cause; the issue is almost always the heater element, relay, or wiring. Only consider PCM replacement after thorough testing of all other components.
If the issue is fixed, the code may clear after several drive cycles, but it's often necessary to reset the code with a scan tool to turn off the check engine light immediately.