Passing emissions....
 

My uncle has a '96 Subaru that failed. I told him to get a new O2 sensor and maybe change the egr valve. I heard someone here say they dumped something in the tank to pass.

??

I think it all depends on why it failed. Did he get a written report on the various parameters and which ones it failed?

It depends. What did it fail on? High HC could very well be oxygen sensors, since they have a finite life. Usually after about 60,000 miles they are ready for replacement. They don't fail all of a sudden, but gradually read leaner than the actual value. This causes the computer to put in more fuel resulting in higher HC and worse fuel economy. They can get pretty bad before they set off the CEL.

OTOH, NOx is usually from a fault in the EGR system. CO is often cats but can be other things too.

Is he in PA too? You should be able to get the specifics of what failed. Also, was it a computer test (reading the car's onboard diagnostics) or a tailpipe test? (not sure what PA does)

There are ways to tweak the results, but you really have to know what you're trying to tweak... and it's usually just as cheap to fix the actual problem than to rig the test. (and ethically sound)

It won't make up for a bad part, but the stuff your looking for is called "guaranteed to pass". just pour it in the tank and burn up that whole tankful, refuel and go get your test again!:D

I'll ask him for specifics, but all he said in the email is they told him he needs a new catalytic converter and an O2 sensor.

I have heard that putting a spark plug anti-fouler on the O2 sensor will cause it to read cleaner. I do not know the specifics.