This document is an official retarder troubleshooting DTCs procedure covering diagnostic trouble code 0122-05 (Med/High Retarder current low) for Caterpillar 3406E, C-10, and C-12 on-highway truck engines. It details the conditions that generate the code, the system response from the Engine Control Module (ECM), and the required test steps to resolve the fault.
Official retarder troubleshooting DTCs document details
| Brand | Caterpillar (CAT) |
| Model coverage | C-12 Truck Engine 9NS (9NS00001-UP); also applies to 3406E, C-10 engines (8YS, 5DS, 1LW configurations) |
| System / Group | Engine Retarder / Compression Brake Solenoid Circuit (SMCS 1129-038) |
| Publication no. | RENR1328-08 |
| Year / Revision | Published 01/10/2008, updated 16/10/2008 |
| Pages | 1 |
| File size | 0.1 MB |
| Format | |
| Language | English |
Document table of contents
- Conditions Which Generate This Code — ECM detection of low current (open circuit) on cylinders 1, 2, 5, and 6 solenoids
- System Response — Diagnostic code logging, snapshot trigger, check engine lamp behavior, retarder deactivation
- Test Step 1 — Retarder (Compression Brake) Solenoid Circuit – Test procedure
Applicable truck models
This procedure applies to Caterpillar truck engines in the 3406E, C-10, and C-12 families with serial prefix configurations 8YS, 9NS, 5DS, and 1LW. These powertrains were installed across a wide range of Class 8 on-highway trucks. The C-12 9NS configuration is the primary model referenced. Workshops servicing any of these engine platforms that encounter DTC 0122-05 related to the medium or high retarder position should reference this document. For additional technical resources, visit Caterpillar’s official support page.
Who needs this retarder troubleshooting DTCs manual
Diesel mechanics and workshop owners diagnosing engine retarder faults on Caterpillar 3406E, C-10, or C-12 truck engines need this procedure. It is essential when the check engine lamp illuminates and the ECM reports a low current condition on the med/high retarder solenoid circuit, indicating an open circuit that prevents the compression brake from activating.
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