Powertrain, generic · Computer and auxiliary outputs

P0640 Intake Air Heater Control Circuit

Check engine lightOn
EmissionsRelevant
Drive cycleRequired
Limp modeNo

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.

Likely causes

Heater relay or switching module faulty
Control wire or connector open or corroded
Heater grid burnt out or shorted to ground
Heater main cable or fuse loose or faulty
Glow plug or engine module faulty

Symptoms

  • Check engine light
  • Poor cold start in low temperatures
  • White smoke, rough running after cold start
  • Flat battery if the relay sticks

Repair estimate

DifficultyMediumDIY possible
Parts and labour€60–500
Time0.5–2.5 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

intake air heaterintake air heater relayglow plug control module

Related codes

P0380P0670P064CP0110P0541P0542

References: law.cornell.edu, gsi.ext.gm.com, gsi.ext.gm.com, static.nhtsa.gov.