Faulty intake air heater element or internal open circuit; Failed intake air heater relay or control module; Damaged, corroded or loose wiring and connectors in the heater control or power feed circuit; Blown fuse or fusible link in the heater power supply; Poor earth connection on the heater or relay; Fault inside the engine control unit output driver; Outdated control unit software where the manufacturer has issued a fix for this code on that model.

P0640

Intake Air Heater Control Circuit.

Generic
Powertrain
Generic code
Low severity

How this code is built

P: SystemPowertrain
0: Defined bySAE, the same on every car
6: AreaComputer and output circuits
40: FaultThe specific fault in that area

What it means

P0640 is the generic code for a fault in the intake air heater control circuit. It is mostly found on diesel engines, where the heater warms the intake air to help cold starting and reduce white smoke. The engine control unit sets the code when the voltage or current it sees on the heater control circuit does not match what it commanded, which points to an open circuit, a short, or a failed driver or relay.

Symptoms

Malfunction indicator lamp on, and possibly the glow plug or engine warning lamp
Hard starting or longer cranking in cold weather
White or grey smoke from the exhaust on cold start
Rough running or misfire for the first minute after a cold start
Intake air heater not warming up, or staying on when it should not
No noticeable symptoms in warm weather

Possible causes

Faulty intake air heater element or internal open circuitlikely
Failed intake air heater relay or control modulelikely
Damaged, corroded or loose wiring and connectors in the heater control or power feed circuitlikely
Blown fuse or fusible link in the heater power supplypossible
Poor earth connection on the heater or relaypossible
Fault inside the engine control unit output driverrare
Outdated control unit software where the manufacturer has issued a fix for this code on that modelrare

How to diagnose

Read all stored and pending codes and note the freeze frame data, especially coolant and intake air temperature, battery voltage and engine state when the code set
Check the relevant fuses and fusible links for the intake air heater circuit, using the vehicle wiring diagram as the layout varies by model
Visually inspect the heater, relay, harness and connectors for heat damage, corrosion, chafing, water ingress and loose terminals
Check battery condition and charging voltage, as low voltage can cause circuit faults on the heater supply
Using a scan tool with bidirectional functions where available, command the heater or relay on and listen or feel for relay operation
With the heater commanded on, measure the supply voltage, the control signal and the earth at the relay and heater connector against the wiring diagram
Check the heater element resistance and current draw against the manufacturer's specification, which depends on the engine
Check the wiring for continuity and for shorts to earth or supply between the control unit, relay and heater, and then check for any applicable software updates

How to fix

Replace the intake air heater element if it is open circuit or outside specification
Replace the faulty heater relay or heater control module
Repair or replace damaged wiring, terminals and connectors, and treat any corrosion
Replace the blown fuse only after finding and correcting the cause of the overload
Clean and tighten or repair poor earth connections
Replace or reprogram the engine control unit only after the wiring, relay and heater are proven good, or update the software if a known fix applies

Can I keep driving?

Low severity

The car can normally be driven, as the heater only assists cold starting. Book it in soon, because starting may get worse in cold weather and a short circuit could blow fuses or damage wiring.

Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.