P20A4Reductant Purge Control Valve A Stuck Open
- 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
- Stuck open means the purge valve stays in, or leaks into, its emptying position while the system is supposed to dose. Part of the pump output then flows back toward the tank, so working pressure builds slowly, collapses after the pump stops or needs a high pump duty to hold. No manufacturer documents P20A4 publicly; GM, Volkswagen and Hino describe only circuit faults and the opposite failure, the valve that does not move into purge (P20A1, P20A5). Bosch's workshop instructions for the Denoxtronic 2.1 or 2.2 show how such a fault looks: if pressure falls below 4.5 bar within about 120 s after a pressure test without any visible leak, a leak inside the supply module at the 4/2-way valve is suspected and the supply module is repaired. The consequence is poor dosing and rising NOx.
- What you may notice
- Warning lamp on, often with low DEF pressure codes
- Slow pressure build-up after start
- Higher NOx, possibly SCR efficiency codes
- Likely causes
- LikelyWorn or contaminated valve seat leaking internally in the supply module
- PossibleUrea crystals holding the valve in the purge position
- RareValve energized permanently by a driver or wiring fault
- 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: €300 – €1,600
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.