The wonders of computers

[PaulGover]

Yesterday morning I started out having left the car on the drive for the past two days. Very soon, "RESTRICTED PERFORMANCE" message on the dash. Restarting didn't sort it. The engine was OK up to about 2000 rpm, and then it started misfiring (not easy on a Diesel!); ease off and it would change up, and run smoothly again. With care, it would get into top, meaning 70mph was attainable.

First guess was a failed wheel speed sensor; the RH rear one failed about a year ago, and produced at least some of the same symptoms. Whatever, I got to my first way-point. Three hours later, we set of for the second half of the journey. "RESTRICTED PERFORMANCE" and "DPF FULL"! Wow, the DPF hasn't given problems in years, as I drive a lot of miles on the motorway or A34; indeed my previous journey had been a 20 mile dash at about 70mph all the way. Still, driving gingerly we made progress. Then the "RESTRICTED PERFORMANCE" disappeared, a bit like the Cheshire Cat. Then a few miles later, the "DPF FULL" vanished! This was after perhaps 20 minutes on the road.

I think I know what's going on; plugging a UDB-II scanner in wouldn't help. It's all down to COVID!

At the start of the year during lockdown, the car sat idle for a fortnight, and the battery died. I had an old one in the garage (I think the original one from 2008 that I replaced probably 3 or 4 years ago) sitting on the charger, so I swapped it and the car started. The same happened a couple more times, with battery swaps curing the problem, leaving me with the old battery in the car.

It had given no problems until now, the first really cold night this year. And I'd left the dashcam plugged in by mistake. At a guess, the old battery has a duff, or at least marginal, cell; perhaps it's down on water. Whatever, I suspect the dashcam flattened that cell, so the battery voltage was a bit low, but enough to start. But the various computers in the X350 are notoriously picky for battery voltage, so the car started throwing random errors, resulting in the two messages. Then as the voltage recovered on the long drive home, they vanished, leaving only a grin.

I'll clear the UDB-II error logs, and swap the later battery back. At a guess that should be OK unless we get another cold period intersecting with a long break in driving. And of course I won't leave the dashcam mounted!