P20EFSCR NOx Pre-Catalyst Efficiency Below Threshold (Bank 1)
- Generic OBD-II code · Powertrain
- Turns on the check-engine light
- Matters for the ITP
- Can trigger limp mode
- Needs a drive cycle to confirm a fix
- How the car detects it
- P20EF concerns the front stage of an SCR arrangement with two stages on bank 1: the pre-catalyst no longer removes enough NOx. CARB allows catalysts in series to be monitored individually or together; this code is used where the front stage is checked separately, which requires a NOx sensor between the two bricks. Paccar's patent US11976582 describes such a layout and computes the front stage's efficiency from NOx entering and leaving it. Hino uses P20EF on 2016–2019 trucks with J08E and J05E engines; bulletin SB-18-024 corrected the workshop troubleshooting for it, without public thresholds. A failing front stage shifts load to the main catalyst, so overall NOx may still look acceptable at first. P20EE covers the main catalyst, P20ED overheating of the pre-catalyst. The driver mainly sees the MIL.
- What you may notice
- Malfunction indicator lamp on
- Higher NOx at low exhaust temperatures, for example after cold starts
- Likely causes
- LikelyFront DEF dosing valve clogged, crystallised or leaking
- PossibleNOx sensor before or between the stages reading wrong
- PossiblePre-catalyst thermally aged or poisoned
- RarePoor DEF quality
- Inspection (ITP)
- Matters for the ITP. While the check-engine light is on, the car fails the emissions check.
- Repair
- Hard · Usually a job for a workshop.Typical cost: €100 – €3,000
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.