Failed Ontario emission,high HC/CO,low NO.O2 sensor? which brand best?90 740 GL

Imagine - a Volvo dealer that doesn't do "diagnostic tests"!!!! Ha ha ha, really builds confidence doesn't it?

To me your emission numbers look like a clean running, properly tuned car without a catalytic convertor. My guess is you'll need to change the convertor to pass, but honestly there's no substitute for a sharp, experienced mechanic who has the car. He should be able to tell the state of the convertor by seeing how hot it is after normal running.

First off, I'd use the lowest octane gas you can find without ANY additives. Injector cleaner alone will make it rise fast on a test.

Hydrocarbons and CO are the result of bad burning and while the Cat does help a lot, it's not a miracle cure. 87 Octane gas has a higher energy per volume, which means if they run the test on a dyno(engine under stress), the car will be working a BIT less hard than with 94. Especially don't put additives and ethanol mixed gas in.

The low NOx is a sign that the cat is probably in good shape. When mine failed, it jumped to over 1000. New cat and it dropped to ~50.

You're not THAT much over, so it's more of a running rich/dirty problem, IMO.

Recommendation:

Basic tuneup - all cars can use a new set of plugs, air filter, and oil change. Less air flow from a dirty filter means the car will run rich. Properly running, it will emit a LOT more NO as a result, but you are way under the limit there. You can afford to run with a leaner ratio(more air).

On plugs, I recommend a plain high-quality plug. I'm not a fan of the Platinum or splitfire or simmilar types. The ones with a big, fat center piece and two outside ones(forget the technical terms right now) are that I use. Holes or whatnot in the components means it wears out MUCH faster. Two, though, is nice. I've seen ones with four, but that seems overkill.

O2 sensor. If the sensor is $$$, and not the older screw-in giant spark-plug type, do the other things first and test. If it is a normal easy to install/buy type, replace it. Mine was $30 last time.

Run the car at highway speed for 20 minutes or so before the test. You want the engine as warm and optimal as possible, so that when it starts up, it doesn't run rich for the first few miles.

So 87 Octane is the way to go?/

I will post back later if it's the cat(Mike Froebel) or o2 sensor(everybody else cos of low No #s).First off,tune up already d> >>ASM 2525 failed. Curb Idle passed

Absolutely. The H&N actually restrict flow, believe it or not, in most designs, especially if re-oiled. This might account for some of it, if the air flow isn't as "clean" as you'd like.

Changed oxygen sensor.Expensive.But passed. HC now at upper limit allowed i co down to .32 NO increased to 191 but way below limit of 777 Thanks to everybody who responded. Tom