Our ISB in a 2000 BB showed check engine on a fairly regular basis. This light was often accompanied by an engine de-rate until it was cleared.
Ours was caused by a faulty crank position sensor. I also cleaned the multi-pin connector near the engine.
The CPS was about $60 and I changed it in a day or so.
Found post from last year:
"I replaced the crank sensor today.
Your bus will probably be a bit different.
Disconnect neg at batt.
Disconnect neg. connections at buss bar near starter.
Loosen or remove starter, 10mm 12 pt. is required.
If not removed, pry starter away from sensor and remove sensor from block.
Unplug sensor and re-plug new one.
Replace all that was loosened or removed.
I needed a 10mm 12pt. box ratchet to remove the top starter bolt. There are 3.
I had to unplug the multi-pin connector and move the dipstick tube to see the bolt.
Next time I can do this in a couple hours, this was the first time and it took me all day at a 72 y/o's pace.
Road test to come.
This bus would produce a code after being fully warmed, shut off, then re-started. The code would show as SES after 1/2 hr. or so of running FULLY warmed. The engine would de-rate to lower power and the idle had a nasty surge. It would clear after a few seconds to a few minutes., usually instantly via the Scangauge. "
Don, Mary and Spooky the cat.