I have an '86 240. approx 200,000 miles, runs ok, could be better but lots of power. has failed emissions test in the oxides of nitrogen dept. Also has bad exhaust smell like burning paint. Usually able to fix most things but cannot figure this one. Gone to 3 mechanics one says air mass unit needs replacing (hope not) other says cat converter. another says adjustment. Can't afford more mechanics or dealership diagnosis. Can I establish if o2 sesor ok by disconnecting? What are reasonable steps to diagnose. I have already established fuel pressure ok to factory spec. new plugs,cap, wires, etc. timing fine, air filter fine, no vaccuum leaks, injectors fine. I would value any feedback. Thanks

 

You can monitor the O2 sensor with a digital Volt meter.
Connect one lead(positive) to the single wire on the o2. The Black lead to ground and see what ot does.
it should sweep from o to .10 I believe it is. Depending on the meter it might show and look like 0-1 volt.
If it seems to stay on the high side then it is running rich. If it seems to be low then it is running lean.

Unplug the Mass Flow sensor and see if it runs better if it deos then the Mass Flow is bad.