Hi Will,

I appreciate the comments.

I see black soot around the tail pipe with no visible black exhaust, and get
lousy gas mileage so maybe the O2 sensor is off.
Where do I probe the voltage? I'm assuming it needs to remain plugged in.

>You should also check for intake leaks.
You mean vacuum leaks, can I use a vacuum gauge to check vacuum health?
any pointers?

> and fix any exhaust leaks. I don't hear any tell-tale noise in the exhaust

You suspect the AMM is bad, I have a 740 AMM in my parts stash here,
dunno if it's compatible with 1993 B230FT: Bosch 0 280 212 016
That number is not shown in my Volvo pocket data book.
The book says look for 2.5-4.0 ohms between pins 2 & 3
The spare reads 3.9 ohms, I'll measure the one in the car tomorrow.

Is there a circuit board or anything serviceable under the cover with 4 flathead
Phillips screws. I don't want to break a possible good spare to investigate.

See my reply to Porkface where today I popped a RPM sensor code after a stall,
A6 2-1-4 I didn't disconnect battery, just cleared the code.

Thanks, Bill