P20A4 code meaning
Reductant Purge Control Valve A Stuck Open
Powertrain system Generic code P20xx codes Check engine light
What P20A4 means
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.
P20A4 at a glance
- Check engine lightYes, usually on
- Emissions testCan fail
- Limp modePossible
- Repair difficultyHard
- Typical labor1 to 3 hours
- Clears after repairAfter a drive cycle
Symptoms of P20A4
- Warning lamp on, often with low DEF pressure codes
- Slow pressure build-up after start
- Higher NOx, possibly SCR efficiency codes
Common causes of P20A4
- Most likelyWorn or contaminated valve seat leaking internally in the supply module
- PossibleUrea crystals holding the valve in the purge position
- Less likelyValve energized permanently by a driver or wiring fault
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Reductant purge valve
What each part of P20A4 means
| P | Powertrain system: engine, fuel system, ignition, emissions and transmission. |
| 2 | Generic code (second SAE range): it means the same on every make. |
| 0 | Area: fuel and air metering and auxiliary emission controls. |
| A4 | The exact fault in that area. |
What to do next
- Note P20A4 and any other codes stored with it: several codes together often point to one cause.
- Check for loose connectors and damaged wiring around the parts the code names.
- Check open recalls for your car: some faults are fixed for free by the dealer. Enter your VIN.
- Clear the code only after the repair. If it comes back, the fault is still there.
Buying a used car? Scan it for stored codes before you pay. A recently cleared memory can hide a problem.
Questions about P20A4
What does P20A4 mean?
P20A4 means "Reductant Purge Control Valve A Stuck Open". Stuck open means the purge valve stays in, or leaks into, its emptying position while the system is supposed to dose. It is a generic code, so it means the same on every make.
Is it safe to drive with P20A4?
Drive gently and only to a workshop. With P20A4 the car can go into limp mode (reduced power) to protect itself.
Will P20A4 fail an emissions test?
Yes, it can. P20A4 is emissions-related, so most inspection programs fail a car while it is stored or while the check engine light is on.
How hard is P20A4 to fix?
Difficulty: hard. A typical repair takes 1 to 3 hours of work. This is a job for a workshop.
Will P20A4 clear itself after the repair?
After the repair the car has to complete a drive cycle (a mix of city and highway driving) before the test runs again. Until then the readiness monitor shows "not ready", which can fail an inspection.