Intake Air Heater Control Circuit Malfunction
The P0640 code indicates a malfunction in the intake air heater control circuit. This heater is used to warm the intake air in diesel engines for improved cold-start performance. The Powertrain Control Module (PCM) detects an open or short circuit in the heater relay or element, or a voltage mismatch. This can lead to hard starting in cold weather and increased emissions.
01. Most Common Causes
- Faulty intake air heater relay
- Open or shorted heater element
- Blown fuse
- Wiring or connector issues
- Faulty PCM
02. Symptoms You Will Notice
- Hard starting in cold weather
- Check Engine Light on
- Rough idle
- Excessive smoke on startup
03. Step-by-Step Diagnosis
Step 1: Scan for codes and record freeze frame data.
Step 2: Visually inspect the intake air heater relay, fuse, and wiring for damage.
Step 3: Test the heater element resistance with a multimeter (specs vary by vehicle).
Step 4: Check relay operation by applying power and ground; listen for click.
Step 5: If all pass, test PCM output signal to the relay.
04. Commonly Affected Makes
05. Frequently Asked Questions
Is it safe to drive with this code?▼
Yes, but you may experience hard starting in cold weather. It's best to repair soon to avoid potential engine damage from repeated hard starts.
How much does it cost to fix?▼
Typically $50-$150 for parts and labor, depending on the cause. A relay or fuse is cheap; a heater element may cost more.