Error Code P0640 is defined as Intake Air Heater Control Circuit. This means there’s an abnormal input voltage within the intake air heater control circuit, likely caused by a dirty filter or faulty air heater relay.

This code is a generic trouble code, which means it applies to all vehicles equipped with OBD-II system, especially those made since 1996 up to present. This includes vehicles from, but not limited to, Buick, Chevrolet, Ford, GMC, Mercedes Benz, Opel, etc. Specifications on the definition, troubleshooting, and repairs, of course, vary from one make and model to another.

When Error Code P0640 is stored, that means the PCM (powertrain control module, also known as ECM or engine control module in other vehicle makes) has determined a fault a fault in the intake air heater control circuit.

The intake air heater is usually made up of a heater element that works when battery voltage is applied. Increase the temperature of the intake air stabilizes fuel atomization, helping evaporate any moisture that may be present. Warmer intake air can also promote better fuel efficiency.

The intake air heater element is a stationary metal grid that becomes red hot when voltage from the battery is applied. As its glow red hot, the intake air passes through is significantly heated.

PCM uses data input coming from the IAT (intake air temperature) sensors to determine whether the intake air heater is working properly. Intake air heater is limited. Vehicles that uses intake air heater usually have an IAT sensor before and after heater element, allowing the PCM to monitor the operation of the intake air heater more efficiently.

If the PCM sees no difference in the intake air temperature after it has passed through the intake air heater element, then the Error Code P0640 will be stored in the PCM. In some cases, multiple ignition cycles may be needed to activate the Check Engine light.

Common Symptoms

  • Rough idle on cold start
  • Drivability problems when cold
  • Increase in fuel consumption

In some cases, other IAT codes may be present.

Possible Causes

There are many reasons behind this code, such as:

  • Defective intake air heater relay or fuse
  • Defective or disconnected IAT sensor
  • Burnt intake air heater element
  • Shorted or open connectors or wirings
  • Damaged or bad programming on PCM

How to Check

Diagnosing this code requires several tools, such as DVOM (digital volt/ohmmeter), infrared thermometer, and a source for vehicle information.

Use the vehicle information, search for the TSB (technical service bulletins) that is similar to the vehicle, engine, symptoms, and codes shown. Most manufacturers provide the TSB to diagnose fault codes such as this quickly.

The latter will provide the diagnostic flow chart of this error code, connector face views and connector pin-out charts, wiring diagrams, and component testing procedure/specifications.

Connect the diagnostic scanner to the vehicle’s diagnostic port to retrieve all the stored codes and their pertinent data. Take note of this information, clear the codes, then take the vehicle for a test drive until one of the two scenarios occur:

  1. The code does NOT reset, PCM enters Readiness Mode
  2. Code resets

If the codes does NOT reset, and the PCM enters Readiness Mode, that means the problem is intermittent and must develop further before you can diagnose it more accurately.

If the code resets:

First Step

Check all related connectors or wirings. Check intake air heater power supply fuses and relays. Repair as necessary. If there are no problems found, then proceed to the next step.

Second Step

Next, refer to the diagnostic flow charts, connector face views, wiring diagrams, connector pint-out charts, and component testing specifications/procedures from your vehicle information source to test IAT sensor voltage, signal circuits, and ground.

Third Step

Activate the intake air heater system and find the intake air heater connector. Check for both battery voltage and ground. If there is no voltage detected, follow the manufacturer’s testing procedure in testing the fuses and relays, make sure you refer to the specifications as well. If no ground is discovered, check the right ground junction on the engine block or chassis.

If the circuit is complete (battery voltage and ground), then proceed to the next step.

Fourth Step

Test the IAT sensor connector for the sensor voltage (usually 5V) and ground. If there’s no voltage present, then trace the circuit back to the right PCM terminal connector using the DVOM. If there’s no voltage on this pin, you can suspect a defective PCM.

If you found voltage at the PCM connector pin, then repair the open circuit between the IAT sensor circuit and PCM.

If there is no ground, trace back through the circuit leading to the central ground location and conduct necessary repairs.

If ground and voltage are found at the IAT sensor connector, then proceed to the next step.

Fifth Step

Check the actual temperature of the intake air (before and after the intake air heater element) using the infrared thermometer. The scanner data stream will show the temperature (or degree of voltage) is being input to the PCM. You will find out if the IAT sensor/s are working normally when you compare voltage to the temperature information from the vehicle information source.

If the IAT and all system circuits are proven to be functional, then you can suspect a defective PCM or a programming error.

  • If the scanner shows an irrational IAT (38˚ when the ambient temperature is much higher)
  • Disconnected IAT connector is a common case after air filter element has been checked and serviced.

How to Fix

The most common repair for this is replacing the dirty air filter, though sometimes, you may need to replace one or more intake air sensors. Other common repairs include:

  • Repair or replacement of intake air temperature sensor
  • Replacement of intake air heater relay or fuse
  • Repair or replacement of shorted or open wirings or connectors
  • Replacement or reprogramming of PCM

The most common misdiagnosis of this code is failing to make sure the air temperature sensor is connected correctly.

Also, most people quick to assume for a defective PCM, when PCM hardly ever fails.