P2604
Air Heater A Circuit Range/PerformanceP2604 is a generic OBD-II powertrain diagnostic trouble code: Air Heater A Circuit Range/Performance. It is logged by the engine control unit when the powertrain monitor detects that a specific fault threshold has been exceeded — typically resulting in the malfunction-indicator lamp (MIL / check-engine light) being illuminated.
What P2604 means
P2604 is stored when the control module detects that the intake air heater on circuit A is operating outside its expected performance range. Intake air heaters (also known as glow plugs in intake manifold applications or grid heaters on larger diesels) warm incoming air during cold starts to improve combustion stability, reduce white smoke, and lower cold-start emissions. Circuit A typically refers to the primary heater element or the first bank in a dual-heater arrangement.
A range or performance fault means the circuit is electrically present (not open or shorted to ground) but is not performing as expected. This can occur because the heater element has degraded and is drawing current outside the expected window, because a relay is not switching at the right time, or because the heater monitor circuit is reading an unexpected voltage during the heating cycle.
On most diesel engines this fault will illuminate the MIL and may cause extended cranking time or hard cold starts, particularly in sub-zero temperatures. It does not typically cause a de-rate but will degrade cold-start emissions performance. The glow plug or grid heater relay, wiring, and the element itself should be inspected in sequence.
Common causes
Most-frequently reported root causes when P2604 is logged.
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1
Degraded intake air heater element drawing out-of-range current on circuit A.
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2
Faulty glow plug relay or grid heater relay not switching correctly.
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3
High resistance connection in the heater circuit reducing current flow.
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4
Incorrect replacement heater element with wrong resistance characteristics.
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5
PCM or module timer fault causing premature or delayed heater activation.
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6
Corroded or loose heater circuit wiring connections.
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7
Internal PCM heater monitor circuit fault.
Symptoms drivers notice
How to diagnose P2604
A typical diagnostic flow when this code is present.
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1
Connect a scan tool and record all DTCs and freeze-frame data.
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2
Check live data for intake air heater duty cycle and feedback voltage during a cold-start event.
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3
Measure current draw of the intake air heater circuit A and compare to specification.
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4
Inspect the heater relay for correct operation using a scan tool actuator test.
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5
Check wiring and connections at the heater element for corrosion or high resistance.
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6
Measure heater element resistance and compare to the manufacturer specification.
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7
Replace the heater element or relay as indicated by test results.
Vehicles where we've handled P2604
Platforms in our catalogue with confirmed P2604 coverage.
Related powertrain codes
Frequently asked questions
Will P2604 cause a no-start?
It is unlikely to cause a no-start in mild weather, but in very cold conditions a fully failed intake heater can make cold starting extremely difficult.
Is circuit A the same as the glow plug circuit?
On many diesels, intake air heater circuit A refers to the manifold grid heater relay circuit, not individual glow plugs. Check the vehicle wiring diagram to confirm.
Can P2604 cause engine damage?
Not directly, but persistent hard cold starts from a failed heater can increase cylinder wear over time due to inadequate combustion temperatures.
How do I tell a range/performance fault from an open circuit?
A range or performance code means the circuit is functional but outside expected parameters. An open circuit code would be set if there were no continuity at all.
Disabling P2604 in software
RaceTune can permanently disable P2604 — and any other OBD-II diagnostic trouble code — on every ECU family we support. The monitor is disabled inside the ECU itself, so the fault stops being logged: the warning light stays off and the engine never enters limp mode for this code. The change is tied to your exact software version.
ECUs with a P2604 disable in our catalogue
Confirmed coverage from our recipe database — we support many more families. Upload your file and our identifier will match it automatically.
- Bosch EDC17C50 verified 1 software version
Software modifications affect emissions compliance and are not road-legal in many jurisdictions. RaceTune service files are intended for motorsport, off-road, and export use.
Got P2604 in your scan?
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