Cummins Fault Code 3231 – How to Fix SCR Intake Temperature Sensor Error

Fault Code 3231 on Cummins engines indicates a problem with the SCR intake temperature sensor, a critical component in the aftertreatment system. When this sensor fails, it can trigger SPN 3251 FMI 0, causing derates, check engine lights, and regeneration issues.

What Is Fault Code 3231 on a Cummins?

  • Fault Code: 3231

  • SPN (Suspect Parameter Number): 3251

  • FMI (Failure Mode Identifier): 0 – Above Normal Operating Range

  • Component: SCR (Selective Catalytic Reduction) Intake Temperature Sensor

This code means that the temperature sensor reading at the SCR inlet is out of expected range—either too high or erratic—causing the ECM to flag an error and reduce engine performance.

Common Symptoms of Cummins 3231 Code

  • Check Engine Light is on

  • Engine torque derate or reduced power

  • Failed DPF regen attempts

  • High DEF fluid usage

  • SPN 3251 FMI 0 active on diagnostic tools

Causes of Cummins Fault Code 3231

  1. Defective SCR intake temperature sensor

  2. Wiring harness damage or short circuits

  3. Loose or corroded sensor connectors

  4. Excessive exhaust temperatures

  5. Malfunctioning DEF injector or clogged SCR catalyst

How to Fix SPN 3251 FMI 0 – Step-by-Step

  1. Inspect the SCR intake temp sensor for damage, soot buildup, or loose wiring.

  2. Check wiring continuity and look for pinched or shorted wires.

  3. Use a diesel laptop scanner (like Diesel Tech or Insite) to view live sensor data.

  4. Confirm whether the sensor is reading outside normal exhaust temps (typically 200°F–800°F).

  5. Replace the SCR intake temp sensor if values are stuck, erratic, or missing.

  6. Clear the fault code and perform a parked regen to verify the fix.

Recommended Tools

  • Diesel diagnostic laptop kit with Cummins Insite or Universal Diag

  • RP1210 adapter (e.g., Nexiq USB-Link 2)

  • Multimeter to check voltage and ground

  • IR thermometer or thermal camera for exhaust temperature validation

Pro Tip:

Make sure the DEF dosing system is working properly—an injector spraying too much or too little DEF can trigger false SCR temperature codes.

Summary: Fixing Cummins 3231 Code

If you're seeing Cummins Fault Code 3231 (SPN 3251 FMI 0), it’s most likely an issue with the SCR intake temperature sensor or its wiring. Diagnosing and replacing this sensor quickly helps restore full engine power, ensure proper DPF regeneration, and prevent aftertreatment damage.

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