Cummins fault code 343 is the Cummins ISM CM871 (2007-09) diagnostic reference for Electronic Control Module Warning, Internal Hardware Failure – Bad Intelligent Device or Component, reported as PID S254, SPN 629, FMI 12/12 with an Amber lamp. This 6-page workshop PDF covers the Unswitched Battery Supply (ISX and ISM), giving the fault description, shop-talk notes, component location and the step-by-step troubleshooting tree. It is compiled from the TruckFaultCodes.com fault-code library (Copyright © 2017).
Official document details — Cummins fault code
| Brand | Cummins |
| Model coverage | ISM and ISX CM871 (2007-09) engines |
| System / Group | Unswitched Battery Supply (ISX and ISM) / DTCs |
| Fault identifiers | PID S254, SPN 629, FMI 12/12, Amber lamp |
| Source | TruckFaultCodes.com fault-code library (© 2017) |
| Pages | 6 |
| File size | 0.28 MB |
| Format | |
| Language | English |
Document table of contents
- Fault code header (PID, SPN, FMI, lamp)
- Description
- Shop Talk
- Component Location
- Troubleshooting Steps
Reading Cummins fault code 343
The ECM receives constant voltage from the batteries through the unswitched battery wires connected directly to the positive battery post, plus a switched input through the keyswitch wire; this code can set when battery voltage at the ECM drops too low.
The Cummins Shop Talk section advises confirming the ECM calibration is correct and recalibrating if necessary. Documented possible causes include:
- unswitched power taken from the starter instead of directly from the battery, letting voltage drop during cranking
- resistance in the ECM battery supply positive or negative circuits
Component location manual information
The ECM connects to the battery through the OEM power harness and the ECM battery supply stub; battery location varies by OEM.
Applicable truck models
This document applies to Cummins ISM CM871 (2007-09) engines, and the same circuit description is shared with the ISX CM871 platform where noted in the extract. These engines power many Class 8 highway tractors and vocational trucks built by North American OEMs. Confirm the exact ECM calibration and OEM wiring against the manufacturer records; full engine support is published by Cummins official engine support.
Who needs this Cummins fault code 343 manual
Fleet technicians, independent diesel mechanics and workshop owners diagnosing an active ECM internal hardware failure condition will use this sheet to follow the correct troubleshooting order instead of replacing parts on guesswork.
The full PDF is available in the DiagTechPro manuals library; a free account is required to download it.
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