P2604 — Problem with inlet air heater electrical circuit

The engine's inlet air heater circuit is sending an unexpected or faulty signal, indicating a possible electrical issue. This heater helps warm the air entering the engine for better cold starts. The code appears when the engine control unit detects a problem like a short, open circuit, or sensor fault in this system.

Common causes

The cause is almost always one of a few things. Here they are — P2604 rarely comes from anything outside this list:

• Faulty inlet air heater element
• Damaged or corroded wiring/connectors
• Blown fuse or relay in heater circuit
• Faulty engine control module (rare)
• Poor ground connection

Where to start

In the order a sensible mechanic would work — the cheap checks first, the expensive replacements only after:

1. Inspect and repair wiring and connectors
2. Check and replace blown fuses or relays
3. Test and replace inlet air heater element if faulty
4. Clear the code and test drive
5. Replace engine control module only if all else fails

That is what holds for P2604 in general. From here it depends on your engine — tell me which car it is and what it does, and I will give you the steps for it: real procedures, torque figures and diagrams instead of general advice.