Ok so I failed my smog check today

Hc and nox were VERY high....ill post # in a bit

Car has a new rebuilt motor in it only about 900 miles, new plugs, msd ignition, dc header, msd wires, and a cai. Just replaced the front O2 sensor and there is a pending code for the rear O2, for lack of activity

I need some help here also the cat was replaced a couple years ago (don't mean its not bad tho LOL)

What could the problem be?

I just failed smog as well got my car to pass but was hard. Was your motor burning oil before you replaced it if so then your cat is bad. Also if you are running bigger exhaust that came with a silencer and you got a check with it out it will fail due to the cat is not getting hot enough to convert. You could try what I did not right but I passes I put 91 in and ran down to 1/8 tank then put one bottle of the octain booster and ran it at 4000 rpm the few blocks before smog. Like I said this is a bad idea and not proper but I passes my cat was bad due to a bad valve stem seal burned a bit of oil and killed it in a year. Even tho my motor had 15000 miles on it damn cheap parts.

Your cat is bad or was not sufficiently heated up for the test. RRE sells some high flow cats, you may want to pick one up from there.

Yeah I was thinking cat but just got a pending code for the rear O2 sensor, could the rear o2 not reading right cause high hc and nox?

Cats ain't cheap so I don't wanna put one if I don't have to

As far as it burrning oil prior I'm not 2 sure I bought the car with a bad engine, it spun a rod bearing due to a dented oil pan........but I got recipts from the owner and one was for the cat that's how I know how old it is

The rear O2 just checks the functioning of the O2 sensor. It will not cause the high emissions you are getting.

I know what you mean - I need a cat too. I destroyed mine dumping too much fuel/misfire. Another $150, blah!

Huh........I'm fairly sure that the rear O2 does a little more than just checks the O2.....

No.

Originally posted by Socalk61000
Huh........I'm fairly sure that the rear O2 does a little more than just checks the O2.....

It only checks cat efficiency. Rear O2 has no control over fuel trims.

ok, BUT if the rear O2 isint working it wont know what the cat is doing right?

and if the pcm dont know how the cat is working then its gonna lean the engine out wont it? and the lean cond could cause high nox

aha I don't know why my wording was messed up but I was trying to say that the rear O2 only checks the cat efficiency.

The FRONT O2 sensor is the only O2 sensor that the PCM will use to lean/enrichen the mix. If the rear o2 goes bad the PCM will throw a code.