GRoup:
My '91 240 with 177k miles will probably fail NJ emissions this month, and I would like opinions (facts would be even better) on what to change to get it to pass.
Two years ago at 145K miles it was marginal and was above the readings taken two years before that.
The NOx and HC were just under the limit two years ago.
The CO percentage is less than half the limit and has not increased much.

I hate to change the cat because the car will be for sale in a month.
Otherwise it is cherry and has no other issues.
Runs perfectly, fuel mileage remains the same, smooth idle.
It does have a vacuuum "leak" in the heat/defrost system which allows me to hear one actuator working when I accelerate. But that is only a small "leak" and not worth fixing.

Could it be the O2 sensor? That costs a lot less.
If you think it is the cat, is the one sold by FCP Groton a decent one?
Will it do any good to block off the small vacuum line at the engine to eliminate the "leak"?

Should I put in new plugs? The cap, rotor, and wires were replaced last year.
Should I give it a 20 mile high speed run on the interstate before going to inspection? Premium gas?

Who has facts?

Thanks,
S'V'