What Does P0640 Mean?
The PCM/ECM has detected a fault in the intake air heater control circuit. The intake air heater is used primarily in diesel engines (and some gasoline engines in cold climates) to warm intake air for easier cold starting and reduced emissions. A faulty circuit means the heater may not activate, leading to hard starts in cold weather, white smoke, and rough running until the engine warms up.
Common Causes
30%
Faulty intake air heater relay or control relay
25%
Burned out or failed intake air heater element
20%
Open or shorted wiring in the heater control circuit
15%
Blown fuse or fusible link in the heater power supply
10%
PCM/ECM output driver failure for heater relay control
Diagnostic Steps
1
Check the intake air heater relay fuse — it is typically a high-amperage fuse (30–80A). Replace if blown and check for the cause of the overcurrent.
2
Using the scan tool, command the intake air heater on using bi-directional controls. Listen for the relay clicking and feel/measure for heat at the intake heater element.
3
Test the intake air heater relay — apply 12V to the coil terminals and verify continuity through the switch terminals. Replace the relay if it does not switch.
4
Measure the intake air heater element resistance. A typical grid heater should read 0.5–5 ohms depending on the design. An open (OL) reading means the element is burned out.
5
Check the PCM control wire to the relay. Verify the PCM is providing a ground signal when commanding the heater on. If no ground signal, check wiring continuity back to the PCM.
Estimated Repair Cost
$30 - $500
Parts + labor, varies by vehicle and location
The PCM/ECM has detected a fault in the intake air heater control circuit. The intake air heater is used primarily in diesel engines (and some gasoline engines in cold climates) to warm intake air for easier cold starting and reduced emissions. A faulty circuit means the heater may not activate, lea...
The most common cause of P0640 (Intake Air Heater Control Circuit) is: Faulty intake air heater relay or control relay
Typical repair costs for P0640 range from $30 to $500, depending on the vehicle, location, and whether you do it yourself or go to a shop.
The vehicle is safe to drive. The primary impact is harder cold starts and rough running in cold weather until the engine warms up. In warm climates, the driver may not notice any symptoms at all. Schedule repair before cold weather or if experiencing hard starting.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to P0640 to identify the root cause.
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Quick Info
Category
Powertrain
System
Intake Air / Cold Start System
Difficulty
Type
Generic (SAE)
Recommended Tools
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