P20E8: Reductant Pressure Too Low

What it means

Pressure in the AdBlue/DEF supply line is below the target. Usually a failing reductant pump, a clogged filter, frozen lines, or a leak in the pressurised circuit.

At a glance

• Usually lights the check-engine light
• Affects emissions; the car can fail an emissions test
• The engine or gearbox may go into limp mode

How it works

The reductant pump pressurizes the DEF supply line so the injector can deliver the correct amount into the exhaust for NOx control. This code sets when monitored line pressure stays below the target; a weak pump, restricted filter, frozen line, or pressure-side leak can prevent the system from building pressure.

Common causes

Most likely first.

1. Reductant pump weak or failed
2. Reductant filter clogged with crystals
3. Leak in pressurised reductant line

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. Read freeze-frame data and compare commanded and measured reductant pressure with a scan tool to see whether pressure fails to build or drops during operation.
2. Inspect the pressurized DEF lines, connections, and pump area for leaks, damage, crystallized deposits, or signs of freezing.
3. Check DEF condition and the serviceable filter, if fitted, for contamination or crystallization that could restrict flow.
4. Check the pump connector, wiring, and power supply for damage or poor connections.
5. Run the manufacturer’s specified pump and pressure tests, taking appropriate care because the system is pressurized.
6. If pressure remains low despite a sound pump supply and clear lines, test the pressure sensor and its circuit to the manufacturer’s specification.

Common fixes

• Repair a leaking or damaged pressurized reductant line or connection.
• Thaw and inspect a frozen line, then repair any damage and address the cause of freezing.
• Replace a restricted serviceable reductant filter and clean crystallized deposits as specified by the manufacturer.
• Repair the pump wiring or connector if testing finds a supply fault.
• Replace a reductant pump confirmed to be weak or failed.

Can I keep driving?

Avoid unnecessary driving and arrange prompt diagnosis, especially if power is reduced or a starting countdown is displayed. Low DEF pressure can disable effective NOx control and may lead to further power limits or a no-start restriction.