Cummins fault code 1678 is the official Cummins troubleshooting manual for the Aftertreatment Diesel Exhaust Fluid Tank Temperature Sensor fault on ISX11.9 CM2250 engines. This 11-page PDF documents SPN 3031, FMI 3, and the amber-lamp response, and it walks through the circuit description, the guided diagnostic steps, and the exact conditions for setting and clearing the code. The aftertreatment diesel exhaust fluid temperature sensor monitors the temperature of the DEF inside the tank. The DEF controller reads the signal-pin voltage, converts it to a temperature value, and reports that value to the engine ECM.
Official document details
| Brand | Cummins |
| Model coverage | ISX11.9 CM2250 and ISX12 CM2250 (2010–2012) |
| System / Group | Diagnostic Trouble Codes (DTCs) — Aftertreatment DEF Tank Temperature Sensor |
| Fault code | 1678 (SPN 3031, FMI 3) |
| Fault description | Aftertreatment Diesel Exhaust Fluid Tank Temperature Sensor — Voltage Above Normal or Shorted to High Source |
| Warning lamp | Amber |
| Reference | Cummins INSITE service tool; Procedure 019-032 (ECM Calibration Code) |
| Pages | 11 |
| File size | 0.2 MB |
| Format | |
| Language | English |
Cummins fault code 1678 table of contents
- Overview and circuit description
- Component location
- Conditions for running the diagnostics
- Conditions for setting fault code 1678
- Action taken when the fault code is active
- Conditions for clearing the fault code
- Shop Talk service notes
- Guided troubleshooting steps and associated procedures
Applicable truck models and engines
This document applies to Cummins ISX11.9 CM2250 and ISX12 CM2250 engines from the 2010–2012 range fitted in heavy-duty on-highway trucks. The SPN 3031 / FMI 3 definition and the guided steps follow the same fault-code logic Cummins publishes for its CM2250 platform, so always confirm the ECM calibration on the vehicle against the current revision using the Cummins INSITE electronic service tool. Verify engine coverage on the Cummins official website before ordering parts.
Who needs this Cummins fault code 1678 manual
Any technician or workshop owner diagnosing Cummins fault code 1678 on an ISX11.9 CM2250 engine needs this procedure to work through the fault correctly. A high-side voltage fault typically means an open return or a signal wire shorted to a voltage source, so the manual routes you through pin-to-pin harness checks before condemning the sensor. The manual ends with the associated-procedures table and the “Reset All Faults” clearing steps so the repair can be validated.
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