Powertrain, generic (extended)

P20A4 Reductant Purge Control Valve A Stuck Open

Check engine lightOn
EmissionsRelevant
Drive cycleRequired
Limp modePossible

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.

Likely causes

Worn or contaminated valve seat leaking internally in the supply module
Urea crystals holding the valve in the purge position
Valve energized permanently by a driver or wiring fault

Symptoms

  • Warning lamp on, often with low DEF pressure codes
  • Slow pressure build-up after start
  • Higher NOx, possibly SCR efficiency codes

Repair estimate

DifficultyHardWorkshop job
Parts and labour€300–1,600
Time1–3 h

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

Parts involved

reductant purge valve

Related codes

P20A5P20A1P20E8P24DF

References: law.cornell.edu, mediathek-bosch-automotive.com, picoauto.com.