This MX SCR catalyst efficiency diagnostic manual provides detailed troubleshooting procedures for fault code P3774, which indicates that the NOx conversion across the SCR catalyst is lower than expected. Published by PACCAR as part of their 2013 MX Diagnostic Service Manual, this document guides mechanics through systematic diagnosis of SCR catalyst performance issues in EPA 2013-compliant engines.
The manual covers the complete diagnostic workflow, from initial visual inspection through advanced DAVIE testing procedures, enabling workshop technicians to identify root causes and restore engine performance.
Document details and specifications — MX SCR catalyst
| Brand | PACCAR |
| Model coverage | PACCAR MX engines (EPA 2013) |
| System/Group | DTCs / MX Engines (EPA 2013) |
| Publication number | P3774 |
| Year/Revision | 2013 |
| Pages | 3 |
| File size | 0.3 MB |
| Format | |
| Language | English |
Table of contents
- Fault code description and information
- Component locations and diagnostic conditions
- Set and reset conditions for P3774
- Possible causes and additional information
- Step-by-step diagnostic procedure (Steps 1–7)
- Visual inspection protocols
- DAVIE direct testing procedures
- Verification drive cycle requirements
Applies to these models
This diagnostic reference applies to PACCAR MX engines meeting EPA 2013 emissions standards. It covers the Selective Catalytic Reduction (SCR) aftertreatment system, NOx sensors, Diesel Exhaust Fluid (DEF) dosing systems, and the EAS-3 electronic control unit. Mechanics working on PACCAR heavy-duty trucks equipped with MX engines will use this MX SCR catalyst efficiency manual to diagnose and resolve P3774 fault codes and related aftertreatment performance issues. For more information, visit PACCAR official website.
Who needs this SCR catalyst efficiency manual
Diesel technicians, fleet maintenance managers, and independent workshop owners diagnosing PACCAR MX engine faults require this guide. The SCR catalyst efficiency diagnostic procedures are essential for technicians performing aftertreatment system repairs and emissions compliance work.
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