Intake Air Heater Control Circuit
Diagnostic trouble code P0640 indicates that the Powertrain Control Module (PCM) has detected an electrical fault in the intake air heater control circuit. The PCM sets this code when the voltage or resistance in the circuit falls outside of the expected manufacturer specifications, preventing the heater from operating correctly during cold starts.
While the vehicle will typically still run, this issue can cause significantly extended cranking times or a no-start condition in freezing weather. It should be diagnosed before the cold season to ensure reliable starting and prevent excessive starter wear.
Common symptoms
- Illuminated Check Engine Light
- Hard starting or extended cranking in cold weather
- White smoke from the exhaust during cold startup
- Rough idle immediately after a cold start
- Reduced fuel efficiency during engine warm-up
Likely causes
- Faulty intake air heater relay
- Defective intake air heater element
- Blown fuse or fusible link in the heater circuit
- Corroded, damaged, or loose wiring and connectors
- Faulty Powertrain Control Module (PCM)
What to check before replacing parts
Inspect fuses and fusible links related to the intake air heater
Look for: Look for blown fuses or burnt fusible links that provide high-amperage power to the intake air heater circuit.
Visually inspect the wiring and connectors at the heater element and relay
Look for: Look for melted insulation, heavy corrosion, or loose connections that could cause high resistance or an open circuit.
Test the intake air heater relay
Look for: Listen for a click when the relay is commanded on, and use a multimeter to verify that battery voltage successfully passes through the load side to the heater.
Measure the resistance of the intake air heater element
Look for: Use a multimeter to check for continuity across the heater element; an open circuit or infinite resistance indicates a burnt-out heater that requires replacement.
Verify the PCM control signal to the relay
Look for: Check for the proper ground or voltage trigger signal from the PCM to the relay control side during a simulated or actual cold start condition.
This page explains the generic P0640 definition and is general reference information, not a diagnosis for your specific vehicle. Actual causes, severity, and repairs vary by make, model, engine, and mileage — always confirm with a scan tool and a qualified technician before replacing parts.
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