Seeing “AdBlue quality” or “start prevented” even after a top-up?
A full drain and refill only helps when the fluid is contaminated or the system has crystallised, not when a sensor or heater has failed.
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
- Warm the van. Coolant at 60 °C thins crystals and speeds draining.
- Remove the fill cap and insert the suction hose to the tank floor.
- Pump out all fluid until the hose gurgles air—about 14 L on most L3 vans.
- Drop the pickup module (usually four 8 mm bolts) and inspect the gauze. Swap the screen if crystals coat more than 25 %.
- Add two litres of de-ionised water. Shake the van or rock it on the suspension to rinse walls.
- Suck the rinse water out until the hose empties.
- Re-fit pickup with a new O-ring. Torque bolts 6 Nm; over-tight cracks the ABS flange.
- Refill with sealed ISO-22241 AdBlue. On Sprinter tanks pour 9 L; Masters and Movanos take 14 L.
- 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.
No pump? No drain can?
Phone 07503 134 362 and book a mobile flush at your depot—most jobs wrap in 40 minutes.
Frequently asked questions
Can I reuse drained AdBlue after filtering?
No. Once exposed to air it absorbs CO₂ and drifts off spec.
Does flushing void warranty?
No manufacturer blocks fluid replacement as long as ISO-grade AdBlue is refilled.
What if the pump won’t prime after refill?
Run three key-on cycles. If pressure stays at 0 bar the pump seal dried out—replace or rebuild.
