This story will take a little time. Grab a seat.

I have a 94 740iL that recently failed its emissions check. I had 2 places tell me two different things regarding the fix. One said taht everything from the exhaust mainfold to the muffler needed to be replaced...right. Shop #2 told me the drivers side O2 sensor was bad and probably the associated cat was no longer effective. They wanted a ton of $ to fix it, so I ordered the O2 sensor and an aftermarket cat ("guarnateed" to pass 50 state emissions tests) and had a local muffler shop install them.

I went back today for another emissions check and the result was WORSE than the previous test. It had failed the unburned hydrocarbons (HC) test during the first run. The limit is 0.8 grams per mile (GPM) and it blew 1.15 GPM initially. Today it blew 1.70 GPM! Almost 50% worse than the already out of spec output. Carbon monoxide reading also went up but is still below the acceptable limit.

I have noticed that since the parts were installed the car idles a bit rougher at a slightly higher RPM. The "check engine" light is not coming on though so I have to figure the computer likes what it sees. The high HC output makes me think the car is running richer than before, but I can't figure out why it would be worse with a new sensor.

Anyone ever run into this with their car? Any advice form the masses on what my next step should be? Think a "simple" tune up could fix her up? Suggestions appreciated.

Thanks,
spmoore39@yahoo.com

Which driver side sensor did you replace the post or pre cat sensor? also for shits and giggles did you should replace the passenger side. Lastly are you sure there are no leaks fore the precat sensor

Did you take the car out and run it at a good hiway speed for about 20 minutes before going to the emissions station?
It is getting cold where you live and if the car has not been driven it will produce much higher reading then it would on a good 90 degree Chicago day.
Just a thought and it might get you by.

I'd double check that O2 sensor and/or the wiring. If your readings got worse and the car ran rough soon after the O2 was put on.

Might have a vacuum leak too. If the idle is higher and rough, maybe someone didn't re-connect something when they put the car back together. Check all of your vac lines and sensor connections.