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Discussion Starter #1
Ok my car kept throwing the P0420. After futher investigation I found out that the CAT O2 sensor was bad. I replaced it. But didn;t reset the code yet. My question is do I reset the codes or will it reset itself with a few drive cycles?? if I remember correctly that code will not reset right away unless you clear it manually. Is my memory serving me well ?? :D
 

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13 Challenger RT, 11 CTS Coupe Premium, 94 Eldorado (RIP)
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Yes, it does serve you well. Even when a code is posted, the PCM still tests that particular sensor. When the sensor gives passing parameters, the code will remain in history for a particular number of ignition cycles before it disappears.
 

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Eldyfig said:
Yes, it does serve you well. Even when a code is posted, the PCM still tests that particular sensor. When the sensor gives passing parameters, the code will remain in history for a particular number of ignition cycles before it disappears.
Exactly as Eldyfig said, the code will stay in memory as a "history" code, meaning that the code happened in the past and is not currently a problem. It won't hurt anything to leave the code in there. As long as the sensor is replaced, you are back to normal.
 

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Discussion Starter #4
mcowden said:
Exactly as Eldyfig said, the code will stay in memory as a "history" code, meaning that the code happened in the past and is not currently a problem. It won't hurt anything to leave the code in there. As long as the sensor is replaced, you are back to normal.
I understand about the code in history, ,however what about the MIL light?? Will it clear after a few drive cycles?? It should clear when that code becomes a history code.
 

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STSMAN said:
I understand about the code in history, ,however what about the MIL light?? Will it clear after a few drive cycles?? It should clear when that code becomes a history code.
If the SES light is still on, you may have a CURRENT code in there in addition to the HISTORY P0420. Have you checked for codes in every category, or just waited for all of the categories to scroll by? If there are no other codes, then clearing the P0420 will snuff the SES light. If you do find another code, let us know so we can help you figure it out.
 

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Discussion Starter #6
mcowden said:
If the SES light is still on, you may have a CURRENT code in there in addition to the HISTORY P0420. Have you checked for codes in every category, or just waited for all of the categories to scroll by? If there are no other codes, then clearing the P0420 will snuff the SES light. If you do find another code, let us know so we can help you figure it out.
no just the current P0420 code.. but mind you I just repaired the car today.
I need to complete more drive or ignition cycles. The light should go out after 3 of these cycles, we will see.

Thanks for the help
 

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STSMAN said:
no just the current P0420 code.. but mind you I just repaired the car today.
I need to complete more drive or ignition cycles. The light should go out after 3 of these cycles, we will see.

Thanks for the help
Actually I just looked up the P0420 code and it means "Catalyst System Low Efficiency," which means that the catalytic converter isn't working as it should. Which O2 sensor did you replace? The one before or after the converter? It's possible that the converter is bad and will need to be replaced to make the SES go away. If the code still shows up as CURRENT, the problem is still happening.
 

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If you do not clear it, it will extinguish the MIL and go into history after 3 ignition cycles with no fault detected. It will then go into, and stay in, history for 50 ignition cycles, after which it will be deleted.
 

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You'll probably be looking at that light for awhile unless you clear it.
As Ranger said it must pass the diagnostic test 3 times before the light will go out. The catalyst monitor test might not run every time, in fact it may actually rarely run.

Problem number two is... a bad O2 sensor will not set a P0420. A bad O2 sensor will set a bad O2 sensor code. P0420 indicates that the O2 sensors are in fact good and the catalyst is not doing it's job. The O2 sensor diagnostic tests MUST pass before the P0420 DTC diagnostic will even run. If the PCM knows the O2 sensors are operating properly and the test enable criteria are met the P0420 test will run.


These conditions must be met in order for the diagnostic to run:
No TP sensor, MAF sensor, MAP sensor, IAT sensor, ECT sensor, HO2S, HO2S Heater, Fuel trim, Misfire, EVAP System, EVAP Control, VSS, or Idle Speed DTCs are set.

• Engine speed since end of last idle period is more than 800 RPM for 46 seconds.

• Idle period is less than 180 seconds (3 minutes).

• Actual engine speed is within 100 RPM of desired engine speed (Non-California)

• Actual engine speed is within 125 RPM of desired engine speed (California).

• Engine run time is more than 7.5 minutes.

• The predicted catalyst temperature is between 420°C and 650°C (788°F and 1202°F).

• BARO is more than 75 kPa.

• ECT is between 71°C (160°F) and 120°C (248°F).

• IAT is above -6°C (21°F).

• Engine is in closed loop fuel control.

• Tests attempted this ignition cycle is less than 12.

• Test attempted during idle period is less than 1.
Here are the conditions for the code to clear or the MIL to go off:

The PCM will turn the MIL OFF during the third consecutive trip in which the diagnostic has been run and passed.

• The History DTC will clear after 40 consecutive warm-up cycles have occurred without a malfunction.

• The DTC can be cleared by using the scan tool Clear DTC Information function.

HTH
 

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Read these details and had the cat replaced on my '00 Deville after the P0420 code came on. Code returned! Is it possible one of the O2 sensors crapped out when the cat was bad? Curious if anyone had success fixing the P0420 code with a replaced O2 sensor.
Thanks
 

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prcouture - Re-read what ewill just posted carefully. P0420 is not an O2 sensor code. In fact if thrown it means the O2's are good. K_C
 

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eWill3rd posted that ^^ 2 1/2 years ago...........I believe that you have an after-cat O2 sensor which monitors the after-cat oxygen content, a measure of cat efficiency. If that sensor was removed and replaced during the cat exchange it's entirely possible that an 8 year old wire, subject to some intense heat, has let go. The instructions for that sensor also caution about bending one of the wires which carries a "clean air reference" signal. That after-cat sensor can be found at Lindsay Cadillac over there >>>>> (Parts, Luke) or www.rockauto.com. It is a plug & play item, up on a lift.
 

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prcouture Reread what ewill posted carefully. A P0420 code being thrown actually means the O2's are good. A bad O2 will throw an O2 code. K_C

oops - didn't mean to say that again - looks like sub and me crossed paths
 

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Read these details and had the cat replaced on my '00 Deville after the P0420 code came on. Code returned! Is it possible one of the O2 sensors crapped out when the cat was bad? Curious if anyone had success fixing the P0420 code with a replaced O2 sensor.
Thanks
Hmmmmmmmmmmmmmmmmmm :tisk:

First off did you have a GM converter installed?
Was it one of those aftermarket converters that is 1/2 the size?

The GM catalytic converter, P/N 25755681

Some will also require a new software calibration to keep the converter from failing again!
New calibration to correct the EST control circuit in the PCM from turning off spark intermittently, which causes misfire events, possibly damaging the catalytic converter, as indicated by DTC P0420. Improved EVAP diagnostics and correction for Mode 6.
 

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If you bought an aftermarket cat, take the car back and have them install the right one.
Legally they have to provide you with one that will do the job the GM one did. Many aftermarket ones have trouble meeting the requirements to pass the on board diagnostic tests... which means they don't work.
 

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If you bought an aftermarket cat, take the car back and have them install the right one.
Legally they have to provide you with one that will do the job the GM one did. Many aftermarket ones have trouble meeting the requirements to pass the on board diagnostic tests... which means they don't work.
Who are "They" cuz I recently bought an 02' DTS that's got an aftermarket Cat & I just can't afford the $900+ it cost to replace it with the GM factory Cat. I just had the Cat replaced & the SEL came back on 3 days after I had it replaced. Plz HELP.
Also where are the spark plugs located on the 02' DTS. Thanks.
 

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You'll have to fight out the cat with the selling dealer. If the SES light was on when you bought the car, I have no idea of your recourse. The Northstar series of FWD cars hates aftermarket cats: they supply nothing in the way of reduced restriction (there isn't any to speak of) and almost always seem to cause DTC problems.

The spark plugs are down in wells inside each cam cover, connected by conductive boots to each of 8 coils in the pack cassettes which are set into the cam cover tops. The cylinder heads are technically 4-valve semi-hemi (penthouse) units with the spark plug in the center of the 4 valve array. The plugs are ACDelco 41-987 Double Platinum units (ONLY) and can be obtained from one of several outlets used by most of the CF members: It's all in here and Seville/Deville. 100,000 mile replacement.

(retired QMCM (SS) (DV) COB, all subs)
 

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You'll have to fight out the cat with the selling dealer. If the SES light was on when you bought the car, I have no idea of your recourse. The Northstar series of FWD cars hates aftermarket cats: they supply nothing in the way of reduced restriction (there isn't any to speak of) and almost always seem to cause DTC problems.

The spark plugs are down in wells inside each cam cover, connected by conductive boots to each of 8 coils in the pack cassettes which are set into the cam cover tops. The cylinder heads are technically 4-valve semi-hemi (penthouse) units with the spark plug in the center of the 4 valve array. The plugs are ACDelco 41-987 Double Platinum units (ONLY) and can be obtained from one of several outlets used by most of the CF members: It's all in here and Seville/Deville. 100,000 mile replacement.

(retired QMCM (SS) (DV) COB, all subs)
So these are the only plugs i can use? Cuz I bought 8 of the Bosch Platinum 4. So would u suggest me trying to change them myself or just pay to let a shop do it. The front seems easy but the back would be near impossible. Thanks for sharing the knowledge.
 

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Two pieces of advice.

#1. Don't use Bosch platinum plugs in a GM engine. They won't run right and it may even cause some real drivability issues for you. Look over the forums and read the stories about how people had poor running cars until they put stock plugs back in.

#2. If you don't know where the spark plugs are, I'd recommend you pay someone to put them in. It could be expensive depending on where you live but it might keep you from running into some potentially serious problems.
 

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Two pieces of advice.

#1. Don't use Bosch platinum plugs in a GM engine. They won't run right and it may even cause some real drivability issues for you. Look over the forums and read the stories about how people had poor running cars until they put stock plugs back in.

#2. If you don't know where the spark plugs are, I'd recommend you pay someone to put them in. It could be expensive depending on where you live but it might keep you from running into some potentially serious problems.
I second that

I had a coworker about 10 years ago that had a 67 Camaro (not stock) that ran 12 second 1/4's. He had Bosch Platinum plugs in it. He put them in every car he or his family member had. I informed him that the Bosch plug was not the best for his application.
We did a test.
He pulled the plug and installed AC Delco Rapid Fire plugs. This plug is very similar to the AC Delco Platinum or Iridium plug.
He ran a quicker time at the track and he said that he had better throttle response.

The Bosch plugs he removed had buildup on the porcelain portion of the plug. The design of the plug will allow for this type of build up. A Northstar can burn oil. The mix of those two will lead to a possible misfire or weak spark.

I was a drivability tech for about 7 of my first 12 years. I didn't have a hoist most of the times. I have changed a lot of spark plugs and I can tell you that the Bosch Platinums are on the bottom of my list. I have seen a lot of gimmick plugs over the last 22+ years and them and Split Fires are at the bottom.
 
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