P20A5: Reductant Purge Control Valve Stuck Closed

What it means

Reductant purge control valve A is mechanically stuck closed — DEF lines cannot be vented at shutdown, increasing risk of freeze damage in winter.

At a glance

• Usually lights the check-engine light
• Affects emissions; the car can fail an emissions test

How it works

Reductant purge valve A opens at shutdown to let DEF drain from the delivery lines toward the tank, helping protect them from freezing. P20A5 indicates the valve is mechanically stuck closed, so the lines may not vent when the engine shuts down.

Common causes

Most likely first.

1. Valve stuck closed mechanically
2. Carbon or debris buildup preventing free movement
3. Component wear with age - mechanical play

Symptoms

• Check engine light or NOx warning on
• AdBlue/DEF level or quality warning
• Possible no-start countdown after ignore period
• Failed emissions inspection (NOx)

How to diagnose it

1. Review freeze-frame data and reductant-system status to confirm when the purge valve was expected to open.
2. Inspect valve A and the line connections for DEF crystal buildup, contamination, physical damage, or a blocked vent path.
3. Use the manufacturer's output test to command valve A open and observe whether line pressure or flow changes as expected.
4. Check the valve's movement and the purge path for a restriction that prevents venting, following the vehicle's specified procedure.
5. If the valve and path are free, check its wiring and control command to rule out an electrical fault being reported as a stuck-closed condition.

Common fixes

• Clear a confirmed restriction or DEF crystal buildup from the valve or purge path using the manufacturer's procedure.
• Replace purge valve A if it remains mechanically stuck closed.
• Repair damaged or blocked line connections that prevent the lines from venting.
• Repair the control circuit if testing shows the valve is not receiving the required command.

Can I keep driving?

The vehicle may run, but DEF left in unpurged lines can freeze and damage the delivery system in cold weather. Have it repaired before freezing conditions if possible, and heed any emissions warning or no-start countdown.