P0640 is an OBD-II diagnostic trouble code indicating a malfunction in the intake air heater control circuit. This code is typically found on diesel engines, where the intake air heater warms incoming air to improve combustion during cold starts. The PCM (Powertrain Control Module) has detected an open, short, or invalid signal in the circuit that controls the intake air heater.
The relay that supplies power to the intake air heater has failed, either due to worn contacts or internal electrical failure. This is one of the most common causes of P0640.
Damaged, corroded, or broken wires in the intake air heater control circuit can cause open or short circuits, triggering the code. This is common in vehicles exposed to moisture or road salt.
The heating element itself may have failed, causing an abnormal resistance reading that the PCM interprets as a circuit malfunction.
A malfunctioning IAT sensor can send incorrect signals to the PCM, causing it to incorrectly diagnose a heater circuit problem.
Oxidation, moisture intrusion, or loose pins in the connectors for the intake air heater circuit can cause intermittent or continuous circuit failures.
In rare cases, the Powertrain Control Module may fail to properly control the heater circuit. This is the least likely cause and should be diagnosed only after all other possibilities are ruled out.
While the vehicle may still run, you will likely experience poor cold-start performance, rough idle, and increased exhaust emissions. In cold weather, the engine may be difficult to start. It is advisable to repair this code reasonably soon.
PCM failure is extremely rare for this code. Most shops report that PCM replacement is rarely needed for P0640 - the problem is almost always in the relay, wiring, connectors, or heater element itself. Replacing the PCM without proper diagnosis often wastes money.
Yes, P0640 applies specifically to diesel engines that use an intake air heater system. Gasoline engines do not have this component, so this code would not appear on gas-powered vehicles.
While not an emergency, this code should be addressed before cold weather arrives. Persistent operation without repair can lead to catalytic converter damage from unburned fuel, excessive wear on the starter from repeated cold-start attempts, and failed emission tests.