P0640Intake Air Heater Control Circuit
- Generic OBD-II code · Powertrain
- Turns on the check-engine light
- Matters for the ITP
- Needs a drive cycle to confirm a fix
- How the car detects it
- 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.
- What you may notice
- Check engine light
- Poor cold start in low temperatures
- White smoke, rough running after cold start
- Flat battery if the relay sticks
- Likely causes
- 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
- RareGlow plug or engine module faulty
- Inspection (ITP)
- Matters for the ITP. While the check-engine light is on, the car fails the emissions check.
- Repair
- Medium · Often possible to do yourself.Typical cost: €60 – €500
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.