When to Drain and Re-Fill the AdBlue Tank – Full-Flush Procedure

The AdBlue tank is meant to be a fit-and-forget component: top up with ISO-22241 fluid and drive.
Yet fleets that store vans outdoors or buy urea in bulk drums sometimes end up with waterlogged or crystal-filled tanks.
When conductivity drifts or sludge blocks the pickup screen, the ECU logs P20EE efficiency codes and starts a no-start countdown.
A complete drain and refill—commonly called a full-flush—restores concentration and flow without swapping the whole SCR unit.
This guide walks through the signs that you really need a flush, tools required, and the nine-step procedure Daimler, PSA and Iveco outline in their 2024 workshop notes based on the ISO-22241 maintenance annex.
If you prefer to outsource the job, a mobile van can perform the rinse at your depot; see the Stoke service hub for booking.

When a full drain makes sense

Flushing is rarely scheduled; it solves specific contamination faults.

• Refractometer reading below 31 % or above 34 % – dilution or evaporation pushes urea out of spec.
• Brown or milky streaks in the sight tube – diesel splash or rust flakes from drums.
• Blocked pickup mesh every 10 k mi even after filter swaps – crystal snowballing inside the tank walls.
• Repeated P20E8 low-pressure codes with a healthy pump – sludge restricting the internal swirl pot.

If none of these appear and the injector doses cleanly, a simple top-up beats a full drain.
But once concentration veers off spec, dosing maths breaks and the ECU over- or under-sprays urea.
Left unchecked it ends with P204F “SCR efficiency below threshold” and a locked starter.

Tools and safety kit

You need:

• 12 V diaphragm transfer pump rated for urea
• Two metres of 12 mm clear hose and shut-off tap
• 20-litre translucent drain can marked “DEF ONLY”
• Five litres de-ionised rinse water (not tap water)
• New pickup screen (cheap insurance) and O-ring kit
• Nitrile gloves, eye shield, absorbent mats

AdBlue is non-toxic but corrodes bare steel and stings cuts; gloves keep hands out of the stuff and mats stop urea bleaching tarmac.

Full-flush procedure

1. Warm the van. Coolant at 60 °C thins crystals and speeds draining.
2. Remove the fill cap and insert the suction hose to the tank floor.
3. Pump out all fluid until the hose gurgles air—about 14 L on most L3 vans.
4. Drop the pickup module (usually four 8 mm bolts) and inspect the gauze. Swap the screen if crystals coat more than 25 %.
5. Add two litres of de-ionised water. Shake the van or rock it on the suspension to rinse walls.
6. Suck the rinse water out until the hose empties.
7. Re-fit pickup with a new O-ring. Torque bolts 6 Nm; over-tight cracks the ABS flange.
8. Refill with sealed ISO-22241 AdBlue. On Sprinter tanks pour 9 L; Masters and Movanos take 14 L.
9. Prime the system. Key on for 30 s, off 10 s, repeat three times. Live pressure should hit 4.5–5 bar then drop to 3.2 bar stable.

Road-test for five kilometres at 2 000 rpm cruise; P20EE pending flag should flip to “test passed.”
If pressure hiccups, swap the in-line filter then clear codes again.

Log it in your maintenance file

Record fluid batch number, temperature and refill volume on the SCR maintenance checklist.
ISO-22241 calls for traceability whenever fluid leaves its original container.

Frequently asked questions