Changed oxy sensors still get P0420

Changed oxy sensors still get P0420

Postby JuneauCabbie [OP] » June 11th 2011, 2:59am

This is driving me nuts! When I purchased my '03 Safari last November, it appeared that the entire exhaust system had been replaced --- when we had it up on the lift during the pre-purchase inspection, the exhaust pipe, cat converter, and muffler all looked new, with no dirt or rust.

I knew that it needed a tune, and had the plugs, plug wires, cap, and rotor replaced about 2 months after I bought it. During the winter, I spent time idling because you can't stay warm in a taxi at 10 degF if you can't run the heater.

I began having a problem with the "check engine" light coming on --- my wrench put his scanner on it, got a code P0420, reset it, and it would be fine for a week or so. Then it began happening all the time, whenever the engine was warmed up and I would be driving at 55mph or so for 10-12 miles, it the "check engine" light would come on with at Code p0420 when I would stop and idle at a stoplight.

I got an Ultragauge and was able to clear the code, but I've been doing it every time I stop and idle. We replaced the Bank 1 oxy sensors but that didn't fix the problem.

Any clues about this run at 1600-2000 rpm for 10-12 miles and get a P0420 when you stop and idle at 600 rpm? Should I get the exhaust analyzed? Should I be checking something other than oxy sensors?

FYI gas mileage and performance have improved with the new oxy sensors.
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Re: Changed oxy sensors still get P0420

Postby Leeann_93 » June 11th 2011, 3:19am

Yeah, you need to check the engine coolant temperature sender - the one that talks to the PCM, not the one that talks to the gauge.

Your wrench should be able to view what it's saying on his scan tool. If it's off from the gauge, it probably needs to be replaced.
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Changed oxy sensors still get P0420

Postby 1997funbox » June 11th 2011, 5:15am

P420 is usually a bad catalytic assembly :(
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Re: Changed oxy sensors still get P0420

Postby NY Safari TONKA » June 11th 2011, 6:27am

You only have one O2 sensor on your System? :rolling:
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Re: Changed oxy sensors still get P0420

Postby JuneauCabbie [OP] » June 11th 2011, 8:36am

Leeann_93 wrote:Yeah, you need to check the engine coolant temperature sender - the one that talks to the PCM, not the one that talks to the gauge.

Your wrench should be able to view what it's saying on his scan tool. If it's off from the gauge, it probably needs to be replaced.


My Scangauge says the coolant temp is 194.6 when the engine is warmed up. Since it connects to the ODBC connector, isn't it reading the sensor that talks to the PCM?
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Re: Changed oxy sensors still get P0420

Postby JuneauCabbie [OP] » June 11th 2011, 8:39am

NY Safari TONKA wrote:You only have one O2 sensor on your System? :rolling:


No. If you read what I wrote a little closer, my wrench changed BOTH Bank 1 sensors.
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Re: Changed oxy sensors still get P0420

Postby Leeann_93 » June 11th 2011, 1:47pm

JuneauCabbie wrote:My Scangauge says the coolant temp is 194.6 when the engine is warmed up. Since it connects to the ODBC connector, isn't it reading the sensor that talks to the PCM?


Then the ECT is fine - forgot you had a ScanGauge.

He replaced both upstream O2 sensors? The usual culprit in the P0420 is the downstream O2 sensor (bank 1 sensor 3), not the upstream.

Other culprits are: exhaust leak prior to the cat and a bad cat (along with bad ECT).
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Re: Changed oxy sensors still get P0420

Postby NY Safari TONKA » June 11th 2011, 7:26pm

JuneauCabbie wrote:
NY Safari TONKA wrote:You only have one O2 sensor on your System? :rolling:


No. If you read what I wrote a little closer, my wrench changed BOTH Bank 1 sensors.



Sorry incomplete question...You saying both...what about Bank 1 No.3 sensor also change?..and all wires on them are good and O2s are not coming out?...Do you here like a Thin(sink)..noise after van its turned off?
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Re: Changed oxy sensors still get P0420

Postby paulo57509 » June 26th 2011, 8:37pm

Your '03 Safari has three O2 sensors; Bank 1, Bank 2 and a post-cat.

It's not clear, but it appears that the OP is stating that both the Bank 1 and post-cat sensors were changed. If so, I agree with the others that this might be a bad cat issue.

The ECM looks at the upstream sensor (Bank 1 and 2) wave forms and the downstream sensor (post-cat) wave forms and compares the two. The upstream sensor wave forms should fluctuate while the downstream wave form should remain pretty steady. If both wave forms are similar, P0420 code is set.

You might check for an exhaust leak upstream of the cat.

Edit: OP mentions that the code is set after running for a while and then idling. Sounds like a weak or defective cat.
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Re: Changed oxy sensors still get P0420

Postby paintdrying » June 26th 2011, 10:09pm

I wonder if long periods of idling with a rich mixture may have killed the cat. I have heard cats do not die, they are murdered.
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Re: Changed oxy sensors still get P0420

Postby JuneauCabbie [OP] » July 2nd 2011, 9:16am

Been gone, stopped checking for updates while the site was down...

1. No exhaust leaks. None. The exhaust system was new when I purchased the van, and my mechanic carefully checked for any leaks.
2. The cat converter seems to be working properly. My wrench doesn't have an exhaust gas analyzer available, but there is no hydrogen sulfide rotten egg smell or acid sulfite odor from my tailpipe. Spark plugs were not fouled.

But the problem is still happening. Don't have a scope available to check waveforms.
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Re: Changed oxy sensors still get P0420

Postby JuneauCabbie [OP] » July 27th 2011, 8:23am

Well, I got the answer I didn't want! Finally took Taku 73 to our local muffler shop, on the premise that if you want answers in a specific area like the exhaust system, go to the expert. I've known Brillo, the owner, since I was dating his older sister years ago, and he knows his stuff. He's the local Flowmaster shop.

He didn't charge me to run his scanner on my rig and let me look at the waveforms produced by the 3 oxy sensors. His conclusion was that the replacement exhaust system installed just before I bought the car has an undersized cat converter. It's marginal, operating just fine when the rig is warming up, or while I'm cruising on the expressway, but failing when it's hot and I'm idling at a stoplight after cruising at 55 or 60. I can get away with resetting the check engine light with the ScanGuage until I can actually afford to replace it without causing any damage to the engine (no emissions testing requirements here).

It won't be cheap to replace it with a robustly sized one, and I'll have him do it instead of my regular wrench, who has been blowing me off when I want these questions answered in spite of the substantial amount of money I spend at his shop. If I could find a better shop that would give taxis priority (which he does because he knows our rigs are our livelihood), I'd go there, but haven't found one yet.

I'm still suffering the trauma from the retirement 2 years ago of the mechanic I had for 40 years, a man I could always count on to do whatever was needed, and work with me to make it affordable, and that was before I was driving my own cab. When my income totally depends on my vehicle, having a good wrench that can take care of problems instantly is critical.
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Re: Changed oxy sensors still get P0420

Postby rev_les » July 28th 2011, 4:30am

Aside from the chance that the converter is dead, make ABSOLUTELY SURE that your wrench is using Delphi, Delco, or Nippon O2 sensors.......and especially NOT A BOSCH sensor. A brand spankin' new Bosch sensor, will continue to throw codes on these vans. I believe somewhere on here another member figured out that the resistance is incompatible with the PCM, but either way, they will NOT operate correctly. ( I have 3 of them I bought prior to learning this that I tossed and replaced with Nippon's, and the problem miraculously went away.
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Re: Changed oxy sensors still get P0420

Postby redfury » July 28th 2011, 5:02am

rev_les wrote:Aside from the chance that the converter is dead, make ABSOLUTELY SURE that your wrench is using Delphi, Delco, or Nippon O2 sensors.......and especially NOT A BOSCH sensor. A brand spankin' new Bosch sensor, will continue to throw codes on these vans. I believe somewhere on here another member figured out that the resistance is incompatible with the PCM, but either way, they will NOT operate correctly. ( I have 3 of them I bought prior to learning this that I tossed and replaced with Nippon's, and the problem miraculously went away.


I'm running Standards in my van with no issues, and try to find non Bosche O2 sensors around here! I'd have to order Delco's.
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Re: Changed oxy sensors still get P0420

Postby Corsemoto » July 29th 2011, 4:57pm

rev_les wrote:Aside from the chance that the converter is dead, make ABSOLUTELY SURE that your wrench is using Delphi, Delco, or Nippon O2 sensors.......and especially NOT A BOSCH sensor. A brand spankin' new Bosch sensor, will continue to throw codes on these vans. I believe somewhere on here another member figured out that the resistance is incompatible with the PCM, but either way, they will NOT operate correctly. ( I have 3 of them I bought prior to learning this that I tossed and replaced with Nippon's, and the problem miraculously went away.


Very good point, last year when doing a 100K service on my '04 Astro AWD, I replaced the three oxygen sensors with Bosch and had it throw codes. Replaced with old sensors, codes disappeared and sent back to Rock Auto (great about returns/credits). I've been getting occasional P0420 so replacing now with Denso O2 sensors (oem), cheaper too.
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Re: Changed oxy sensors still get P0420

Postby MadMid » August 7th 2011, 4:53pm

Was there also. Replaced it all and would still get a 420 after 30 miles or so of driving. Clear the codes and continued on. Couple weeks ago started buying alcohol free fuel. Tank and half later no 420 code. Maybe the fuel, maybe not.
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Re: Changed oxy sensors still get P0420

Postby JuneauCabbie [OP] » August 12th 2011, 10:19am

rev_les wrote:Aside from the chance that the converter is dead, make ABSOLUTELY SURE that your wrench is using Delphi, Delco, or Nippon O2 sensors.......and especially NOT A BOSCH sensor.


The replacement sensor was a Delco.
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Re: Changed oxy sensors still get P0420

Postby rev_les » August 13th 2011, 10:26pm

You should be good on the sensor then. Ironically, a few days ago, a bud changed his O2 sensors on his Town N Country with Bosch....same thing, it started throwing more codes than it had to begin with....I told him about the problems we have on Astro's with them, and my suspicions. He swapped to Densos, original problem AND new problems solved.
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Re: Changed oxy sensors still get P0420

Postby Leeann_93 » August 13th 2011, 10:43pm

My local parts house doesn't even stock Bosch O2 sensors, only Denso. They'll get the Bosch if you insist, but they'll hand you Denso otherwise. They got tired of the returns.
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Re: Changed oxy sensors still get P0420

Postby chevymaherchevymaher is online! » March 22nd 2012, 4:02am

Something I stumbled across. This guy is usually right to.
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