I'm suffering the same dilemma. I called the local Volvo shop, and he explained the emissions test results to me. Mine failed for HC output and NOx output. HC is Hydrocarbons, basically unburned fuel. NOx is a gas that is only really produced when the engine runs hotter. I believe he said when the combustion chamber exceeds an average temp of 2500 degrees F. He also said it would be $115 for them to check it out. I've changed my sparkplugs, removed and cleaned the throttle-body (with carb cleaner and a shop towel). Run a bottle of fuel injector treatment through it, and premium fuel. And given her an oil change and a new harmonic balancer.

However, from my research into further solutions, there is a rebreather hose at the bottom passenger side of the B230 engine, which can cause your emissions to increase if it's disconnected. That's reconnected now. Also, I was told to check the O2 sensor, and catalytic converter.

As for high NOx readings being caused by hotter engines, I don't know if this is true, but my car did overheat in the inspection bay, to the point of boiling over. I thought nothing of it, as the inspection tech had parked it with the nose literally against the closed bay door, but how likely is it that my thermostat has crapped out?

I feel like I'm chasing gremlins with the smallest cartoonish mallot available.
Aaron Casper
1989 240 DL Wagon.