Applicable Systems
EMCP 4.2
Symptoms
SCR fault indicator active; engine derate at level 1 or 2; NOx conversion efficiency below threshold.
Possible Causes
- SCR catalyst degraded or poisoned
- DEF dosing valve clogged
- NOx sensor reading out of range
Troubleshooting Steps
- Step 1: Check NOx sensor readings (inlet vs outlet)
Outlet NOx < inlet NOx (conversion working) - Step 2: Inspect DEF dosing valve for clogging
Valve sprays evenly, no crystals - Step 3: Check SCR catalyst for physical damage
Catalyst intact, no cracking - Step 4: Clean or replace DEF dosing valve
Dosing valve flow within spec - Step 5: Clear code and perform drive cycle for SCR verification
NOx conversion > 70% at operating temp
How to Reset
Clear via Cat ET after SCR system repair and drive cycle.
Common Replacement Parts
- DEF Dosing Valve P/N: 385-7312
- NOx Sensor P/N: 385-7423
Frequently Asked Questions
- Q: Can I bypass the SCR system?
No. Emissions regulations require functional SCR. Bypassing will trigger permanent derate and violate emissions law.
Source: verizonconnect.com DTC guide
Category: Heavy Equipment