I bought a 97 406 2.1TD Executive a few months back and everything has
been peachy until today. A bit of background: It has just over
170,000mls on the clock, averages about 45mpg around town and doesn't
use oil or water. It has been serviced at regular intervals all its
life (it shows). I've been running it on bio-diesel for the past month
without any issues.

I've noticed over the last few days that it has been taking slightly
longer to stop when turning the ignition off. Today came to switch it
off at B&Q and it ran on at tickover for about 4-5 seconds - no reving
on its own and throttle inputs made no difference. When I came out and
restarted it, it did so straight away as usual except after a few
seconds the check engine lamp came on and stayed on.

Now every time I turn the ignition off it runs on for a few seconds and
the engine lamp comes on and stays on whilst driving. It starts without
hesitation (hot or cold) and drives/responds just a usual.
I first suspected a faulty stop solenoid but being fly-by-wire I'm
unsure as to whether it has a conventional stop solenoid. Does anybody
have any ideas?

Your help is much appreciated.

Cheers
Craig