P0640 code meaning
Intake Air Heater Control Circuit
Powertrain system Generic code P06xx codes Check engine light
What P0640 means
Some diesels (for example GM's 6.6 L Duramax LML and the Cummins in Ram trucks) warm the intake air with a heater grid switched by a relay or semiconductor switch. P0640 reports a fault in its control circuit. GM's LML glow plug module expects a digital feedback from the heater while it is commanded on; if it is missing for 2 s (75 % of 4 s), the code sets and the lamp follows after two trips. Current, voltage and temperature faults run under GM's own codes P154A–P154D. CARB requires intake air heaters to be checked for response, current and voltage drop. The result is poor cold starts, white smoke and rough running after a cold start; a relay stuck on drains the battery – Ram recalled 2021 trucks because a shorted heater relay could overheat. Ford's 2023 diesel does not use P0640.
P0640 at a glance
- Check engine lightYes, usually on
- Emissions testCan fail
- Limp modeNot expected
- Repair difficultyMedium, DIY possible
- Typical labor0.5 to 2.5 hours
- Clears after repairAfter a drive cycle
Symptoms of P0640
- Check engine light
- Poor cold start in low temperatures
- White smoke, rough running after cold start
- Flat battery if the relay sticks
Common causes of P0640
- Most likelyHeater relay or switching module faulty
- PossibleControl wire or connector open or corroded
- PossibleHeater grid burnt out or shorted to ground
- PossibleHeater main cable or fuse loose or faulty
- Less likelyGlow plug or engine module faulty
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Intake air heaterIntake air heater relayGlow plug control module
What each part of P0640 means
| P | Powertrain system: engine, fuel system, ignition, emissions and transmission. |
| 0 | Generic code: it means the same on every make (SAE standard). |
| 6 | Area: computer (engine control module) and output circuits. |
| 40 | The exact fault in that area. |
What to do next
- Note P0640 and any other codes stored with it: several codes together often point to one cause.
- Check for loose connectors and damaged wiring around the parts the code names.
- Check open recalls for your car: some faults are fixed for free by the dealer. Enter your VIN.
- Clear the code only after the repair. If it comes back, the fault is still there.
Buying a used car? Scan it for stored codes before you pay. A recently cleared memory can hide a problem.
Questions about P0640
What does P0640 mean?
P0640 means "Intake Air Heater Control Circuit". Some diesels (for example GM's 6.6 L Duramax LML and the Cummins in Ram trucks) warm the intake air with a heater grid switched by a relay or semiconductor switch. It is a generic code, so it means the same on every make.
Is it safe to drive with P0640?
Usually yes, for a short time, if the check engine light is steady and the car drives normally. If the light flashes or the car runs badly, stop and have it checked.
Will P0640 fail an emissions test?
Yes, it can. P0640 is emissions-related, so most inspection programs fail a car while it is stored or while the check engine light is on.
How hard is P0640 to fix?
Difficulty: medium. A typical repair takes 0.5 to 2.5 hours of work. A careful DIY repair is possible with basic tools and a scanner.
Will P0640 clear itself after the repair?
After the repair the car has to complete a drive cycle (a mix of city and highway driving) before the test runs again. Until then the readiness monitor shows "not ready", which can fail an inspection.