03 base auto, 165k miles, P0420

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03 base auto, 165k miles, P0420

Postby BoostFrenzy » Wed Jan 09, 2008 10:38 pm

Just started getting this catalyst inefficiency code... and just a few days after replacing the plugs with OEM lasers, PCV valve and oil change... does this ALWAYS mean the cat has failed or has anyone falsely gotten this code due to bad front/rear O2 sensor?

Diagnostic troubleshooting path? Drivability is perfect


-2008 Corvette Coupe MN6, tuned & tinted... more mods to follow
-2007 Mazda3 S-touring hatchback 5spd AUTO, stock & tinted. GF's
-2003 Pontiac Vibe base AUTO, stock and winter duty (185k miles)
gone but not forgotton...
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Re: 03 base auto, 165k miles, P0420 (BoostFrenzy)

Postby joatmon » Wed Jan 09, 2008 11:07 pm

see http://forums.genvibe.com/zerothread?id=19728

Mine was the cat, I don't think there has been anyone who consistently got the P0420 and had it caused by something other than the cat. It could happen, but I don't remember an instance where it wasn't the cat

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Postby BoostFrenzy » Wed Jan 09, 2008 11:09 pm

bummer, what's the cheapest replacement option? it's built into the midpipe and dealer wants $1000 which isn't happening, the car will be replaced with a 09 vibe in 1-2 mos

-2008 Corvette Coupe MN6, tuned & tinted... more mods to follow
-2007 Mazda3 S-touring hatchback 5spd AUTO, stock & tinted. GF's
-2003 Pontiac Vibe base AUTO, stock and winter duty (185k miles)
gone but not forgotton...
02 Z06, 04 & 05 STi, 93 & 94 RX-7 TT, 89&90&91 Audi 80/90, 02 A4
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Postby BoostFrenzy » Thu Jan 10, 2008 4:19 am

also, are the OEM oxygen sensors bosch or denso?


-2008 Corvette Coupe MN6, tuned & tinted... more mods to follow
-2007 Mazda3 S-touring hatchback 5spd AUTO, stock & tinted. GF's
-2003 Pontiac Vibe base AUTO, stock and winter duty (185k miles)
gone but not forgotton...
02 Z06, 04 & 05 STi, 93 & 94 RX-7 TT, 89&90&91 Audi 80/90, 02 A4
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Re: (BoostFrenzy)

Postby Mavrik » Thu Jan 10, 2008 5:01 am

P0420 is usually always a failure with the cat itself. a cheap replacement would be anything other then OEM converter.

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Postby drunkenmaxx » Thu Jan 10, 2008 5:08 am

i would trust the code. my maxx threw a CEL light the other day. got it scanned, came back as a bad thermostat. $10 later, CEL turned off on its own after a few minutes.
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Re: (drunkenmaxx)

Postby BoostFrenzy » Thu Jan 10, 2008 5:42 am

isn't quite that simple for a P0420 code, see the P0420 indicates a catalyst inefficiency and what that means is

1) the cat *IS* bad
2) front O2 is bad and "thinks" it's targetting stoich and can't, therefore the cat can't keep up and mixture remains too rich
3) rear O2 is bad and isn't stable at the expected delta difference between the front and itself, but still working good enough to trigger a bad cat code and not a bad O2 sensor code

So either way, the "right" way to fix this is new OEM O2 sensors and cat, but as that's extremely expensive, replacing only the bad part would be optimal in her situation.

So, looks like i'll order both O2 sensors first since they've never been replaced and see where i'm at then.... MPG hasn't been stellar anyway, getting 26.8mpg average @ 75mph (4spd auto)


-2008 Corvette Coupe MN6, tuned & tinted... more mods to follow
-2007 Mazda3 S-touring hatchback 5spd AUTO, stock & tinted. GF's
-2003 Pontiac Vibe base AUTO, stock and winter duty (185k miles)
gone but not forgotton...
02 Z06, 04 & 05 STi, 93 & 94 RX-7 TT, 89&90&91 Audi 80/90, 02 A4
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Re: (BoostFrenzy)

Postby RIT » Thu Jan 10, 2008 10:35 am

For $75, your dealer will tell you that it is the cat. MIne was replaced under warranty. Good luck, but it is almost always the cat. Why not just disconnect the battery for 2 minutes to clear the code before you trade it in.

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Postby drunkenmaxx » Thu Jan 10, 2008 9:42 pm

when my CEL came on, i did a ton of reading about them. it was noted that the O2s get a bad wrap and are commonly blamed, when they are in fact a very durable part that rarely goes bad. many people replace them, get the code cleared, then think everything is ok. then, the light comes on again up to months later and people think the sensor went bad again. it becomes a cycle when in fact the original problem was never fixed. the system calculates tons of crap and it can take several instances of being + or - out of range of what is considered normal before the CEL throws.
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Postby joatmon » Thu Jan 10, 2008 10:50 pm

The cat's in these cars seem to be of a finite lifespan, don't know how many miles are on your girlfriend's vibe. Probably the cat, and the cheapest alternative is probably to get some aftermarket thing welded in.

Hard to spend a lot of money on something she is going to get rid of real soon, maybe RIT is right.

If the cat is bad, then it will tank your MPGs, and if you replace it with a low cost performance cat, your MPGs will be better than anything possible in an 09 Vibe.

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Re: (joatmon)

Postby drunkenmaxx » Thu Jan 10, 2008 11:01 pm

165k miles, the cat being bad isn't a ridiculous notion!
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Postby BoostFrenzy » Fri Jan 11, 2008 6:26 am

It could be surely bad, I just have a hard time accepting a cat failing when operating in a perfectly tuned and maintained environment, it's not like i'm some schmoe that lets her drive it around on 3cyl for a week or something, the car has been kept in tip-top shape mechanically since day 1 so this is depressing... trying to talk her into a 07-08 civic cause the 1ZZ MPG isn't that great anyway

-2008 Corvette Coupe MN6, tuned & tinted... more mods to follow
-2007 Mazda3 S-touring hatchback 5spd AUTO, stock & tinted. GF's
-2003 Pontiac Vibe base AUTO, stock and winter duty (185k miles)
gone but not forgotton...
02 Z06, 04 & 05 STi, 93 & 94 RX-7 TT, 89&90&91 Audi 80/90, 02 A4
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Postby mike7311 » Thu Jan 17, 2008 12:00 pm

I just replaced mine (135k). When the code first came up I ignored it. Then it came on more and more until my MPG dropped by 20%; a 6 month span. Don't waste your money on oxygen sensors, I did and it didn't fix this problem. I now have alot more low and high end power and am back to 32mpg instantly. No more cel. If you live in CA you have to use the stock unit $500 from gm, but aftermarket units are availible for everyone else. I have heard of problems with the aftermarket units though.
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Re: (BoostFrenzy)

Postby spark123 » Thu Mar 13, 2008 3:21 am

Along with either of the O2 sensors being bad there could be low voltage on the O2 sensor heater.

I had the intermittent cel till it finally stayed on. No drivability isues. I replaced both O2 sensors , cut the cat outlet pipe ahead of the secondary O2 sensor and looked it over and it looked normal. At this point the car needed to go for NYS inspection which requires hooking up to the OBD2 port and downloading the data to Albany. I knew this would not pass this way. I didn't want to shell out the $850 (which was the price 2 years ago) to find out the old cat was probably good. So, I bought one of those O2 simulators online and hooked it up. The code was still there! I checked the O2 sensor output via an OBD2 adapter and software for my laptop which I also purchased in this endeavor . There now was no O2 sensor output. So, I grabbed a voltmeter and started checking voltages at the simulator and found no voltage supplying the simulator. The voltage was tied to the secondary O2 heater voltage which was not there. This voltage is supplied by the PCM and it was actually bad and, to boot you would think it would throw a heater no voltage fault.

To summarize check the heater voltage on the secondary O2 sensor as it is important in proper functioning of the sensor.

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