PACCAR MX (2004 Emissions) Fault Code: P171B EGR Flow Response – Decrease too Slow

PACCAR MX (2004 Emissions) Fault Code: P171B EGR Flow Response – Decrease too Slow

SET CONDITIONS:
The PCI ECU (D420) detects that the EGR mass flow decrease is too slow during changes in the commanded EGR mass flow from a high to low rate.

DIAGNOSTIC CONDITION:
This diagnostic runs:
1. When the engine speed is more than 1000 rpm and
2. The engine mode is DPF regeneration or SCR heating or SCR high efficiency or standard.

CORRESPONDING OBD READINESS MONITOR:
EGR system monitor

RESET CONDITION:
This DTC changes to inactive when the fault is no longer detected.

REPAIR VERIFICATION:
1. Clear all DTCs.
2. Test drive or operate the vehicle to meet the following conditions:
– Corresponding readiness monitor displays ‘Ready’
– Diagnostic conditions are met
– set conditions are met
3. Did the DTC come back? (Can be pending, inactive or active)
– No, repair successful
– Yes, repair unsuccessful

POSSIBLE CAUSES:
1. Clogged EGR system
2. Clogged EGR pressure difference sensor venturi
3. Sticking EGR valve

ADDITIONAL INFORMATION:
1. The engine NOx emission is, among others, controlled by varying the EGR flow toward the inlet manifold of the engine.

2. For this diagnostic, the EGR pressure difference sensor (F751) is used to measure the EGR mass flow during changes in the commanded EGR mass flow from a higher to lower flow rate.