Engine light, MOT and the A23 through Norbury
Norbury is inside the ULEZ whichever side of the Croydon and Lambeth boundary your street falls on, so the diesels still in daily use here are largely Euro 6. Many of them depend on AdBlue, and when the SCR system faults, the engine management light usually comes on with it. That matters at MOT time. On modern cars an illuminated engine light is a failure, however well the car seems to drive.
The A23 London Road through Norbury is where many of these faults develop. It's a red route with frequent buses and slow traffic from Hermitage Bridge down towards Thornton Heath Pond, and the stop-start pace keeps exhaust temperatures down. The SCR system works less efficiently when it's cool, deposits form more readily around the injector, and eventually the downstream NOx sensor reports that emissions aren't being reduced enough. Private hire cars working the A23 all day see this more than most.
Side streets tell a similar story. Around the Norbury Cottage Garden Estate and the roads near Norbury Library, streets are 20mph, narrow and parked on both sides, and cars there often do nothing but short local trips. When the car comes in, we scan the engine and SCR modules, look at how the NOx readings respond once the exhaust is hot, and check the injector for crystals. We'll tell you whether cleaning will do or the part needs replacing, and give you a written quote.
If your MOT is coming up and the light is on, get the repair done first. Once it's fixed, we clear the fault, let the system complete its self-checks on the road test and rescan before handing the car back, all under our 3-month workmanship warranty. The drive is about 5.5 miles: London Road to Thornton Heath Pond, across to the Lombard roundabout and along the A236 over Mitcham Common. If the countdown is running, collection is the safer choice, since stopping on the red route isn't an option if the car won't restart.