P0640Generic code

P0640 Code: Intake Air Heater "A" Control Circuit

The P0640 code means there is a fault in the circuit the computer uses to control the intake air heater on a diesel engine. The engine starts harder when cold. The usual culprit is a bad heater relay.

Severity
Low
Can I keep driving?
Yes, no immediate risk
Estimated cost
$40 to $400
Difficulty
Medium
System
Air and Fuel
Applies to
All OBD2 vehicles

Updated Reviewed by Renzo Chacca

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What the P0640 code means

Many diesels warm up the intake air with an electric grid when the engine is cold. The computer switches it on with high current relays and monitors the control circuit. The P0640 code is set when that circuit is not working as expected.

A relay that does not work leaves the engine without help for cold starts. A relay that sticks closed keeps the grid on, drains the battery and can burn the wiring.

If the grid works but the relay does not switch it off, the constant current can melt the power cables and damage the battery. So if you notice a burning smell or hot cables near the intake manifold on a diesel, check this circuit right away.

P0640 code symptoms

  • Hard starting when cold.
  • White smoke.
  • Dead battery if the relay sticks closed.

Likely causes of the P0640 code

  1. Defective heater relay with contacts burned by the high current.
  2. Control wiring between the computer and the module or the relay.
  3. Corroded high current connections. They get hot and lose contact.

How to diagnose the P0640 code step by step

  1. Test the relay.
  2. Measure the grid current. As a guide, 80 to 200 amps.
  3. Check that the relay switches off during the afterheat period following startup.

How to fix the P0640 code

  • Replace the relay.
  • Repair the connections.

Technical reference data

  • Heater grid current: 80 to 200 amps.
  • Reference values: check the manufacturer data for your vehicle.

Brand-specific notes

If you have a diesel pickup, check the heater relay first, because it is the part that tends to fail.

P0640 on Dodge Ram Cummins

This code is well known on the Dodge Ram with the Cummins diesel.

Frequently asked questions about P0640

Can I drive with a P0640 code?
Yes. The engine runs, but it starts harder in cold weather. Fix it soon, because a stuck relay can drain the battery and overheat the wiring.
What happens in winter?
The engine may be hard to start, because the heater is the cold start help. Expect more cranking and some white smoke.
What usually fails?
The relay. Its contacts burn from the high current the grid draws.
How much does it cost to fix P0640?
Between $40 and $400, depending on whether it is only a relay, the connections or the heater grid.
What does the P0640 code mean?
It is a generic OBD2 code that means the intake air heater control circuit on a diesel is not working properly.
What is the most common cause of P0640?
A worn heater relay. If you see a P0640 code with a burning smell, check the wiring and connections for heat damage.

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