I just took my 89' Plymouth Acclaim in for emission testing ...

I just took my 89' Plymouth Acclaim in for emission testing and it failed the following categories. 2500 RPM test CO(%) 12.17, HC(PPM) 3406, and idle CO(%) 1105, HC(PPM) 842. So what could be wrong with the car? I just put a new O2 sensor on it a couple of months ago!

Could be a variety of things. I'm not to familiar with dodge/chrysler engines, but generally it means at least 1 of 2 things, the fuel isn't being burned completely, or the exhaust isn't being catalyzed properly.

Fuel side: Assuming an EFI engine, it could be plugs, timing, ECU, MAF/MAP sensor, wiring problem (specifically to a sensor, but usually that would throw a code), Fuel injectors, low or high fuel pressure, Bad fuel (of course, but make sure you are using the LOWEST octane fuel your engine will take), low compression (from stuck/leaking valve, or worn rings/sleeves), did I leave anything out?

On the exhaust side, mainly just the catalytic converter, though an exhaust leak prior to the catalytic converter (could theoretically cause the problem, but only if something was entering against a pressure gradient... unlikely at best). Remember, if you fed it contaminated or leaded fuel prior, the contaminants can coat the catalytic converter... (aka problem)...

Think that's about it... start checking things off!

Thank you, I just bought new Bosch Platinum Plus 4 spark plugs for it and installed them yesterday, and changed the oil and filter, and added Lucas Oil Treatment. I recently changed the air filter a couple of months ago this summer as well. I was also told by a mechanic friend that to run a couple of bottles of Techron Fuel Injector cleaner through it. He also suggested buying some stuff that helps with Emission testing at the parts store which I did. It says to put in a full tank, then run it almost empty then fill it up and then go test it. The car doesn't really get any highway miles so to speak, so I was told by a neighbor and another customer up at the parts store to go drive it on the highway for a few miles and get it good and hot with highway driving and then go test it immediately afterwards. That should help blow some of that gunk outta my catalytic converter I would think.

Exactly. If there's no damage to the catalytic converter and that's the problem, that would def. help.

I'm not sure about all the additives, sometimes they actually make the reading worse, but it depends on the problem! For example, if there is no platinum left in your catalytic converter, no amount of stuff in the world will let you pass...

OTOH, when you're trying to hit a flying bird you use a shotgun right?