Why AdBlue that sits too long causes problems
Not every diesel in Purley is worked hard. Plenty belong to owners who use them for trips up Purley Way to the shops, a drive out to Riddlesdown Common at the weekend and not much else. Some sit on the drive for a week at a time between trips. That's gentle on most of the car, but the AdBlue system prefers regular use. The fluid in the tank can sit for many months, and AdBlue slowly breaks down with age and warmth, particularly through a hot summer.
As it degrades, its concentration changes, and the quality sensor in the tank notices. The result is a warning that doesn't clear with a top-up, because adding new fluid to old only dilutes the problem. Low use also means more heat-soak cycles for the miles covered, which encourages crystals around the dosing injector, and cars used mainly on the 20mph roads around Reedham and Woodcote rarely get the exhaust hot enough to help.
We check the fluid's quality first, since that decides everything else. If it's out of specification, the tank is drained and refilled with fresh ISO 22241 fluid and the system checked again before we suggest any further parts. Sometimes that alone clears it. We'll also check the dosing injector while we're there, as low-use cars often have light crystallisation too. Where the fluid is fine and the fault remains, we move on to the dosing test, the heater and pump checks and the NOx sensor readings, and you get a written quote before anything is replaced.
From Purley Cross, the route is up Foxley Lane and Banstead Road to the mini-roundabout, then north on the A237 through Wallington to Hackbridge, about 4.5 miles. Purley's hills don't cause AdBlue faults on their own, but a car that won't restart halfway up one is awkward, so if the countdown is running, come in sooner rather than later. Collection can be arranged, and the repair has a 3-month workmanship warranty.