Cummins fault code 2556

What it means

Cummins' description: Engine Intake Air Heater 1 Circuit - Voltage below normal, or shorted to low source.

SPN 729 is Engine Intake Air Heater Driver #1. FMI 4 means the voltage on the circuit is lower than possible for a good sensor, usually a short to ground, a damaged harness, or a failed sensor.

Cummins engines use an intake air heater (grid heater) or glow plugs to help cold starts and cut white smoke. The ECM controls the heater relay and the wait-to-start lamp.

What the lamp means

An amber (yellow) warning lamp means the engine can usually keep running, but the fault should be repaired soon, ideally the same day.

Common causes

FMI 4 is an electrical or signal fault, so start with the circuit:

• Grid heater relay failed or stuck
• Heater element open or shorted
• Corroded heavy-gauge heater cable or connection
• Wait-to-start lamp circuit fault

If the circuit checks out, the reading may be real. Other possibilities: weak batteries can't power the heater; heater relay stuck on (can drain batteries).

What you can check before calling

• Check whether the wait-to-start lamp comes on in cold weather
• Look for burned or loose heater cable connections
• Notice hard starts and white smoke only when cold

Clearing the code without fixing the cause usually brings it right back, and on emissions systems it can restart derate timers.

Engines and sources

Listed by Cummins for: QSB 6.7, QSL 9, QSB 4.5, QSF 3.8, QSG 12. Cummins uses the same fault code numbers across its electronic engines, including on-highway models like the ISB/B6.7, ISL/L9 and ISX/X15, though not every code applies to every engine or software level. Source: Cummins Tier 4 Fault Codes. Always confirm with Cummins service information (QuickServe) for your engine serial number.