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P0640Intake Air Heater Control Circuit

A fault has been detected in the control circuit for the intake air heater, the grid or element that warms incoming air for cold starts.

Definition

Diesel and some cold-climate gasoline engines use an electric intake air heater (grid heater) ahead of the intake manifold, switched by the PCM through one or more high-current relays. The PCM monitors the relay control circuit and, on many applications, feedback from the heater circuit; P0640 sets when the control circuit is open, shorted, or the feedback does not match the commanded state. Symptoms are mainly cold-weather related: long cranking, hard starting, white smoke, and rough running until the engine warms, and on some trucks a stuck-on heater can cause heavy battery drain or dim lights at idle. The MIL illuminates, but warm-weather driveability is usually unaffected.

Common causes

  • Failed intake air heater relay or corroded relay contacts
  • Open or chafed wiring in the relay control circuit from the PCM
  • Burned-out or shorted heater grid element
  • Corroded high-current feed connections at the heater studs or relay
  • Blown fuse or damaged fusible link in the heater feed
  • Faulty PCM driver for the heater relay control

Common misdiagnoses

  • Replacing glow plugs, which are a separate circuit with their own P038x codes
  • Replacing batteries for hard cold starts caused by an inoperative grid heater
  • Replacing the heater grid when only the relay or its control wire had failed
  • Replacing the starter for extended cranking that was a heater-circuit fault

Troubleshooting steps

  1. 1.Command the heater on with a scan tool

    Activate the intake heater output and listen or measure for relay click and a voltage drop of roughly 1-2 volts at the battery as the high-amperage grid loads the system.

  2. 2.Test the relay control circuit

    Verify the PCM switches the relay control wire (typically grounding it) when commanded, and check for battery voltage on the relay coil's feed side with the key on.

  3. 3.Check the heater element

    Measure the grid element resistance across its studs; most read well under 1 ohm when good, and an open reading condemns the element.

  4. 4.Inspect high-current connections

    Remove and clean the heater feed studs, relay terminals, and any fusible links, looking for heat discoloration and torquing fasteners to specification.

  5. 5.Verify amperage draw

    Clamp an amp probe on the heater feed while commanded on; compare draw to specification (often 90-190 amps on truck grid heaters) to confirm the whole circuit works.

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