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Unread 03-27-2015, 03:37 AM   #1
SSpider
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Check Engine Light?

Maybe someone can steer me in the right direction ... I have a 2005 V-8 4.7 Grand Cherokee with 96000 miles on it ... the idle on it was fluxuating ocasionally up to around 12000 RPM & down to 5000 RPM ... I brought it into a local repair shop in town with a good reputation & they suggested trying first a BG FUEL INJECTION SERVICE they said it cleans everything out & I was told the engine light may come on after a little & to bring it in & they would reset it ... well it came on they reset it & it was back on after about 10 miles so I went back & it now they say it has a P0420 code ... they said it could be a number of different things from plugs to cats to O2 sensors to exhaust or intake leaks & on & on ... I was told it was just a coinsidence? the plugs had 26000 on them so I changed them to give it a shot, after pulling them out they were ready ... went back & had them reset it & it's back on after about 10 miles ... coinsidence? any help/suggestions would be greatly appreciated ...

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Unread 03-27-2015, 03:50 AM   #2
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just did a quick code look up

P0420 - Catalyst System Efficiency Below Threshold Bank 1
Quote:
As the code description implies the P0420 code means that the vehicle's control module has detected that the three-way catalytic converter is not working properly (is not as efficient as the factory is expecting). Replacing the oxygen (O2) sensors may sometimes fix the code, but in most cases the catalytic convert needs to be replaced to fix the problem. If the P0420 code is combined with other codes, try fixing the other codes first.
Have the shop use a scanner to test the O2 sensors to make sure they are working before thinking about replacing the cat.
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Unread 03-28-2015, 03:08 AM   #3
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Thank you Greever I will start there & see if they will scan it for me ... Would you think the FUEL INJECTION SERVICE would have anything to do with the P0420 code? or Just coinsidence it started less then 10 miles after it was done? Thanks again
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Unread 03-28-2015, 05:42 AM   #4
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I would check the bank ONE downstream O2 sensor to make sure it still works. Make sure you replace only with mopar, if a new one is needed.

They cost $38 each at moparpartsdiscounted. Two of mine were bad so I replaced all four. Bank two sensor one is a tough one to change because it is hard to reach.

BTW, I would not count on the fuel injector cleaner to solve your idle problem. Someone needs to put the vehicle on a scanner. The fuel trims at different rpms may give a clue as to what is really going on. The fuel pressure can be checked with a fuel pressure gauge as well as the injectors with a scan tool to determine if each of them is releasing the right amount of fuel.

If you do not have a scanner it may be time to go to the Dealer for a diagnosis. The trusted independent shop offered a poor solution. I would not go back there for this problem.

And, on the rpms you mean 500 to 1,200 right?

Last edited by RCS1300; 03-29-2015 at 09:26 AM.. Reason: Moparpartsdiscounted correction
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Unread 03-28-2015, 05:55 AM   #5
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Id say start with the O2 sensors since they are cheaper (fuel injectors cost $60 a pop i think and more labor intensive to test), but as RCS stated stick with mopar/ntk/ngk replacements. If they do need to be changed, I'd suggest you buy them yourself and give it to the mechanic to install...don't trust them to purchase it because they will probably buy bosch and charge you for mopar.
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Unread 03-28-2015, 08:11 AM   #6
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Thanks again Greever ... at that price I think I'm going to order up the sensors & just change them out & give it a shot ... the shop had said if they do have to replace the cats that they put in new sensors with the new cats ...
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Unread 03-28-2015, 08:39 AM   #7
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Test the O2 sensors before buying them. Most scanners can give you the voltage readings on them. I dont remember off hand, but I know the front O2's (1/1 and 2/1) have an alternating voltage and run between 100 mA and 900 mA i think, and the rear sensors (1/2 and 2/2) have a static voltage. Wouldn't want to change the cats and its just a burnt out sensor, or change the cats and its something else.
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Unread 03-28-2015, 09:23 AM   #8
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Quote:
Originally Posted by SSpider View Post
Thanks again Greever ... at that price I think I'm going to order up the sensors & just change them out & give it a shot ... the shop had said if they do have to replace the cats that they put in new sensors with the new cats ...
My dealer wanted to charge me two hours of labor to change out the 2/1 O2 sensor. I did it myself but had to purchase a 28" long Snap on flat head screwdriver and make a companion tool that would give me the 28" reach to help disconnect and refasten the sensors electrical connection. The other O2 sensors took me 15 minutes each to change out, the 2/1 sensor took me two hours.

Long Screwdriver

http://store.snapon.com/Extra-Long-S...--P647443.aspx


Very hard to imagine the Cats are bad unless someone was driving with the check engine light on for a long, long time.

On the surging idle the O2 sensors are likely not the answer. I would only replace the one that is bad after you test them. The cause could be many things that regulate fuel and air delivery.


> dirty throttle body
> dirty/sticking IAC valve
> low fuel pressure from the fuel pump ( can be easily tested with a fuel pressure gauge)
> vacuum leak
> one or more bad injectors (not real likely but possible and can be tested real easy with a fuel pressure gauge and scan tool like Autoenginuity with the Chrysler package, not code reader)
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Unread 03-28-2015, 09:43 AM   #9
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Quote:
Originally Posted by Greever View Post
Test the O2 sensors before buying them. Most scanners can give you the voltage readings on them. I dont remember off hand, but I know the front O2's (1/1 and 2/1) have an alternating voltage and run between 100 mA and 900 mA i think, and the rear sensors (1/2 and 2/2) have a static voltage. Wouldn't want to change the cats and its just a burnt out sensor, or change the cats and its something else.
this^ Greever is on point! Keep is updated with how the repair goes, i'm glad you're doing it yourself. Always good to learn, and save mulah along the way Also, the first post says your idle went up to 12000 rpm..... haha. Not meaning to nitpick or anything.
Update this post after the new sensors! Good luck
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Unread 03-28-2015, 10:35 AM   #10
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Lol to many zero's 500 to 1200 RPM
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Unread 04-01-2015, 06:59 AM   #11
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Okay so here I go with an update ... I brought it to a local jeep dealer that was recomended to & have them scan & diagnos it ... they came up with the cat being bad in bank 1 ...

I was also recomended to bring it to a local shop to get a price to have a new cat 1 welded in ... When he gave me the price he asked me why the cat was bad, I said I don't know I guess it's worn out & needs to be replaced, he said that cats don't wear out something made it go bad & it could be a number of things (saying to myself here we go again) but it actually went pretty good, I ended up talking to there main man & he had a ton of knowledge, we discussed all the events that went on, luckily I had the plugs I had just changed out & he said it was burning fine, we talked about how the shop did the BG FUEL INJECTION SERVICE because he thought it was to much of a coincidence the P0420 came on that quick after the FUEL INJECTION SERVICE, so he asked how long it took them to perform the FUEL INJECTION SERVICE & I told him 15 to 20 minutes he said what? he said that should take 45 minutes to an hour to perform. He is thinking they fed it in to fast making it mess up the cat? He told me to try letting the tank get down add a can of Sea Foam & fill it with High Test run it on the highway for twenty miles & then burn the tank of gas bring it back to him to reset it & see if that works ... How does that sound?

Sorry for all the hot air not easy to explain
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Unread 04-01-2015, 08:31 AM   #12
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Quote:
Originally Posted by SSpider View Post
Okay so here I go with an update ... He told me to try letting the tank get down add a can of Sea Foam & fill it with High Test run it on the highway for twenty miles & then burn the tank of gas bring it back to him to reset it & see if that works ... How does that sound?
Sounds like this new "main" guy at least is not rushing to any conclusions right away. Disappointing that the Dealer said replace the Cat. To me that simply means he charged you to read the codes and he did nothing more to identify the source of the original problem. Bad Dealer for diagnostics.

What you really need is someone that understands engine diagnostics and can test your O2 sensors, examine your fuel trims, and check some basics like injector function and fuel pressure. Something is causing your idle surge problem.

Sure, run some Seafoam through it but realize that will not solve the original problem that led you to the first "shop".

I am not a parts changer and really hate to suggest this but, based on the "shops" you visited, it seems like it may cost you more in time and money to find a good automotive diagnostician than to purchase and change the Bank 1 downstream O2 sensor (after the CAT) with a new Mopar O2 sensor for $38 (cost of part) - assuming you can spin the wrench and borrow the O2 special wrench from Autozone.

Now, the new O2 sensor will not make your idle issue go away but there is an excellent chance, but no guarantee, it will end your run around on the expensive CAT replacement issue and make that check engine light go away. It is possible that that O2 sensor got contaminants in it from the "fuel injector service" that is causing it to read the CAT output incorrectly.
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Unread 04-01-2015, 01:12 PM   #13
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Thanks for the advice RCS I really do appreciate it I'm learning a lot but the hard way ... I didn't mention in my last post that they told me the 02 sensors were okay ... I think I'm going to try the Sea Foam first I don't want to do them at the same time because if it fixed the light I wouldn't know which one fixed it ... Question though why do you suspect the down stream 02 sensor instead of the up stream one? I really know nothing about them but I was thinking maybe the up stream one get more contaminants because they didn't go thru the cat yet ... again I have no clue but just thinking & I'm not questioning your knowledge at all just trying to learn ... Thank you arain
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Unread 04-01-2015, 01:35 PM   #14
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Quote:
Originally Posted by SSpider View Post
Thanks for the advice RCS I really do appreciate it I'm learning a lot but the hard way ... I didn't mention in my last post that they told me the 02 sensors were okay ... I think I'm going to try the Sea Foam first I don't want to do them at the same time because if it fixed the light I wouldn't know which one fixed it ... Question though why do you suspect the down stream 02 sensor instead of the up stream one? I really know nothing about them but I was thinking maybe the up stream one get more contaminants because they didn't go thru the cat yet ... again I have no clue but just thinking & I'm not questioning your knowledge at all just trying to learn ... Thank you arain
I am a 20+ year jeep owner and enthusiast with experience based knowledge, not a professional mechanic.

The downstream O2 sensors primary purpose is to determine whether or not the catalytic converters are working. If the Bank One downstream sensor (after the CAT) is not working correctly it could give a false reading which would lead a non-diagnostician to believe that the Cat needs to be replaced. For example, the CAT could be fine but the O2 sensor contaminated which may throw a code that the CAT is not working. If it were my Jeep I would replace the $38 downstream O2 sensor out of an abundance of caution before I would replace a $400 CAT. At a minimum, switch the two downstream O2 sensors and see if your problem code moves to the other bank.

O2 sensors get dirty from the inside and outside - they compare air from the outside of the exhaust system to air from the inside. Two of mine (10 years and 70,000 miles driven) were probably ruined mostly from driving during winters on the East Coast where the outsides were exposed to an abundance of salt, dirt, grime, etc. That dirt probably clogged the orifices gathering air from outside the system. Those orifices were jet black compared to the white ones on the new O2 sensors I installed.

A CAT can go bad when the fuel mixture to the engine is way out of whack and sends too rich a gasoline mixture (possibly from misfires or poor fuel metering injectors) into the exhaust and it burns in the CAT. That ruins a CAT. That type of rich fuel mixture should throw a code and a check engine light, likely one that blinks. To ruin a CAT one would need to continue driving that vehicle with a check engine light for many miles.

The upstream O2 sensors primary function is to communicate with the pcm (engine computer) and alter the fuel mixture to ensure it is perfect as it enters the engine.
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Unread 04-12-2015, 06:54 AM   #15
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Well RCS you can say I should have listened to you to begin with & replaced the Bank 1 down stream 02 sensor & I probably wouldn't be going thru all this aggravation now the 02 sensor is on order Anyway I wanted to see how the Sea Foam worked, I ran the tank down added a can of Sea Foam & filled it with the High Test & ran it for 30 straight miles on the highway & then finished the tank normal driving, I filled it back up & had the code reset, instead of the light coming on in less than 10 miles in was back on in 43 miles I thought I was golden after 30 miles ... Now I'm hoping the 02 sensor will do the trick ... Thanks again RCS for all your help, I wasn't doubting you at all it was a good test for me to see how the Sea Foam would actually work cleaning the cat & 02 sensors ...
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