my mechanic is puzzled...

[mastershake]

took the alero in today to get the cat converter changed (p0420 code) and had him test the oxy sensors before to make sure the downstream wasnt just being lazy. oxygen sensors turned out to be working just fine. so then he took it around the block to heat the cat up and then scanned it with an infrared temp scanner. that looked normal too. he has no idea why its throwing a p0420 code, and im starting to get pissed LOL

so im taking it back in wednesday to get more tests done (free of charge)

but in the mean time any ideas??? no rotten egg smell or loss of power, but he did pull up a misfire on cylinder 2 which went away

[AlbinoMonkeyRat]

do you have aftermarket headers? do you have an extension on your O2 sensor? Is the wire to your O2 sensor touching the exhaust at all (headers, pipe, cat, etc)?

is it possible that one sensor is old and one new so that one working more efficiently (even though both are in acceptable spec)?

At this point, I would venture a guess that it's all electronic, and your car is otherwise working just fine.

[mastershake]

no. no, and no. and i was thinking that same thing (because i changed the front o2 sensor and not the rear)

[[ion] C2]

this is engine related, why not post in the subsection that fits

from your previous posts, it seems you have a 3400. it helps in diagnosis to know what we're working on here..

[heineck]

cant you move it? now that ur a mod n all

[mastershake]

well, i thought that since its catalytic converter related (which im pretty sure we all have) i would put it under common problems. but then again i pretty much make all of my threads in this forum :P

but yeah just move it

[Cliff8928]

You're getting the P0420 code because there's too much activity on the downstream sensor...

Originally Posted by GM

A three-way catalytic (TWC) converter controls emissions of hydrocarbons (HC), carbon monoxide (CO), and oxides of nitrogen (NOx). The catalyst within the converter promotes a chemical reaction which oxidizes the HC and the CO that are present in the exhaust gas. This process converts the HC and the CO into water vapor and carbon dioxide (CO2), and reduces the NOx, converting the NOx into nitrogen. The catalytic converter also stores oxygen. The powertrain control module (PCM) monitors this process by using a heated oxygen sensor (HO2S) that is in the exhaust stream after the TWC. This HO2S 2, also referred to as the catalyst monitor sensor, produces an output signal that the PCM uses to calculate the oxygen storage capacity of the catalyst. This indicates the ability of the catalyst to convert the exhaust emissions efficiently. The PCM monitors the efficiency of the catalyst by allowing the catalyst to heat, then wait for a stabilization period while the engine is idling. The PCM then adds and removes fuel while monitoring the HO2S 2. When the catalyst is functioning properly, the HO2S 2 response to the extra fuel is slow compared to the response of the HO2S 1, which is located before the TWC. When the HO2S 2 response is near that of the HO2S 1, the oxygen storage capability and efficiency of the catalyst may be degraded below an acceptable threshold. If the PCM detects the degraded condition, DTC P0420 sets.

[[ion] C2]

Originally Posted by heineck

cant you move it? now that ur a mod n all

Nope I can only play with 2.4L Twin Cam, Forced Induction Performance, and At the Track. I can give infractions and ban everywhere though.

[zzyzzx]

When I replace O2 sensors, I always replace both of them at the same time and use new exact fit Bosch ones. Not suprizingly, I have never had a problem like the one you describe.

[zzyzzx]

How many miles are on the car? I don't trust O2 sensors past 100K miles.

[a.graham52]

o2 sensor is cheaper then a cat. id throw one at it

[comanche]

The P0420 code is usually accurate.

Your mechanic should know what he's doing, so ask him to explain to you the performance of the downstream sensor.

The purpose of the downstream sensor is to determine whether or not the converter is doing its job.
A steady .45 volts from the rear O2 sensor is good.
A range of voltage from the rear O2 sensor indicates converter failure.

I guess you could ask for a graph of the rear O2 sensor... but he should know what he is doing...

[mastershake]

i have 205k miles on my car now.

he hooked a computer up to the OBDII to look at both o2 sensor voltages. both were fluctuating (so the downstream isnt supposed to fluctuate?)

[a.graham52]

no the downstream is not supposed to fluctuate. if it "follows" the upstream sensor then id say the cats bad.

[zzyzzx]

If you have 205K on your car and an original O2 sensor, replace the sensor.

[mastershake]

ok so replace the cat or the sensor? i dont have enough $ for both xD

[a.graham52]

without seeing the graph of the o2 sensors i couldnt tell you wich. but if you saw it... the upstream o2 should bounce up and down quite a bit while the rear should be almost steady and smooth. if it bounces like the front then the cats bad.

[mastershake]

yeah i saw the graph and the downstream definitely bounces around like the upstream. so im going to go ahead and replace the cat

[mastershake]

ok so i go into the autoparts store today trying to buy a cat and the guy asks me why im replacing it. i tell him because the downstream sensor is fluctuating along with the upstream one. he proceeds to tell me its supposed to do that. im like, wtf. im so confused...

[Ryan from Ohio]

I dont believe all the advice given in parts stores...

I find Advance and AutoZone hire just about anyone. So Im sure its similar across the world.