Powertrain, generic (extended)

P20F3 SCR NOx Pre-Catalyst Efficiency Below Threshold (Bank 2)

Check engine lightOn
EmissionsRelevant
Drive cycleRequired
Limp modePossible

P20F3 states that the SCR pre-catalyst on bank 2 no longer converts enough NOx. It belongs to engines with a separate aftertreatment line per bank and a two-stage SCR in each, with a NOx sensor between front and main brick so that the front stage can be judged on its own, an option CARB grants for catalysts arranged in series. No manufacturer documents P20F3 publicly; Hino's use of the bank 1 code P20EF is the nearest documented case. The multi-stage SCR application US20150096287 explains the logic: each stage gets its own efficiency target, and a weak front stage can be hidden because the rear stage compensates, which is why it needs its own monitor. Typical faults are a blocked front dosing valve or a heat-damaged pre-catalyst. P20F2 covers the bank 2 main catalyst. The driver usually only sees the MIL.

Likely causes

Bank 2 front dosing valve blocked or crystallised
NOx sensor between the bank 2 stages reading wrong
Bank 2 pre-catalyst damaged by heat or contamination

Symptoms

  • Malfunction indicator lamp on
  • Increased NOx after cold starts

Repair estimate

DifficultyHardWorkshop job
Parts and labour€100–3,000
Time1–5 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

scr pre catalystnox sensor

Related codes

P20EFP20F2P20F1P20EE

References: law.cornell.edu, patents.google.com, static.nhtsa.gov.