P0172, P0420

Tiny
CVSUNSHINE14
  • MEMBER
  • 2000 TOYOTA CELICA
  • 4 CYL
  • FWD
  • MANUAL
I have two DTC's P0172 and P0420. I replaced the mass air flow sensor and upstream oxygen sensor. Do you think these codes are relevant to each other, or do you think I need to replace the catalytic converter?
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Friday, May 6th, 2011 AT 11:38 PM

14 Replies

Tiny
WRENCHTECH
  • EXPERT
I would say yes on both counts. The rich mixture probably ruined the cat.
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Friday, May 6th, 2011 AT 11:52 PM
Tiny
MECHTRIX
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Iwould say no the cat is only semi clogged now [with carbon]the po172 code indicates it was running rich wich produces a lot of black powdery soot that clings to and clogs the honeycome like screen inside of your cat ....but the parts you replaced should solve the problem if it does not clean up after a few good runs on the highway'' you may have to take it off the car and soak it over night in some water and laundry soap when you see the code po420'it means that its still operating but just below its specified operating temp if you look inside of it with a flash lite you would see a honeycome like screen that screen under nomal operating conditions will glow chery red wich is what causes a thermal reaction to covert real harmful gasses into way less harmful gasses ...anyway'' carbon seems to stick to the cooler parts of the cat first [the outside edge of the screen ] the slowly works its way twards the middle restricting the flow but the middle portion continues to glow after the repair is made and begins a cleaning effect ...long story short ive seen it take as long as 3 weeks to regain its full capacity but most times less ...hope its the bolt in type and not the welded in type if you plan on cleaning it hers a link funny video on the subjecthttp://www.youtube.com/user/scottykilmer#p/u/13/5icTmYItwiE
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Saturday, May 7th, 2011 AT 1:11 AM
Tiny
WRENCHTECH
  • EXPERT
That code is not set because of temperature. It sets when the front 0/2 and the rear 0/2 start to mirror each other which means the converter is not chemically changing the exhaust. It has to meet a series of criteria before it will set the code so false codes are very rare. I've never heard of cleaning one so I have my doubts about that.
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Saturday, May 7th, 2011 AT 1:18 AM
Tiny
WRENCHTECH
  • EXPERT
I just watched that video and I have to say that has to be one of the worst ideas I ever heard. I DO NOT recommend adding lacquer thinner to your fuel at all.
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Saturday, May 7th, 2011 AT 1:25 AM
Tiny
MECHTRIX
  • MEMBER
Oh you might also need to change the other 02 unit if you get any codes for it in the near future but cross you fingers good luck.
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Saturday, May 7th, 2011 AT 1:30 AM
Tiny
MECHTRIX
  • MEMBER
The cat is all about heat. It vital to its funtion o2 sensors are all about engine manigement the cat does it job only after the engine has expeled its waist there are no dtc's that describe a condition where the cat is operating [above the threshold ] becaus a cat that operates above the threshold but below the melt down piont is not broken. The threshold is the point where the temp gets hight enough to start the coversion procces. Below that' is where it starts clogging or other scary things [somtimes] rendering it non usable. Oh and as far as the 02 sensors go"" they have two difrent jobs " the upsream one is used for feedback to the pcm for fuel control and the downsream one is the one used for monitering the cat its somtime refered to as the CMS or [ catylist moniter sensor] . Yes they are where the codes signals originate as the signals do mirror each other. This takes place about three times then a code is set. But I was refering to condition of the cat in the state I may be in now. Not one thats still funtioning nomally. Hence by the time you see this code its already broken if you trust that the codes are correct and I also do as yes" false codes are rare. But if you have the time to take it out and try to clean it I highly recomend it. As it could save you boocoo bucks but if thats not enough then yes just buy a new one. Lots of luck. Hope this helps
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Saturday, May 7th, 2011 AT 4:28 AM
Tiny
MECHTRIX
  • MEMBER
Dont mark my words but I think lacquer thinner and injector cleaner are made of the same stuff. But at this point I dont think id recommend it eather.
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Saturday, May 7th, 2011 AT 4:46 AM
Tiny
CVSUNSHINE14
  • MEMBER
Thanks. Well I had figured replacing the pre cat o2 sensor was a good idea since I had been running rich and it was completely black. The cat doesnt give off a bad odor or anything. But I dont know if it has to smell bad in order to tell if its clogged/bad.
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Saturday, May 14th, 2011 AT 2:24 AM
Tiny
WRENCHTECH
  • EXPERT
Nothing has indicated that your cat is clogged. Your code is an efficiency code. All that deals with is the state of the exhaust coming into it and compared to the state coming out. If they match, it sets the code.
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Saturday, May 14th, 2011 AT 2:27 AM
Tiny
CVSUNSHINE14
  • MEMBER
Oh yea, after I replaced the mass air flow sensor to fix the running rich problem, it runs perfect again. So basically what im saying is that I don't notice a loss in performance from the p0420 code, EXCEPT when I first start the car after its been sitting overnight, it seems a little boggy, but doesnt bog at all after running for a minute.
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Saturday, May 14th, 2011 AT 2:35 AM
Tiny
CVSUNSHINE14
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Ok, I'll replace the cat and hopefully that fixes it.
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Saturday, May 14th, 2011 AT 2:40 AM
Tiny
MECHTRIX
  • MEMBER
That code is not set when the engine is running efficiently 'its set when it running ineffiently "if it ran better after driving it for a while '' then like I said ''it was only semi clogged not bad yet. And after the repair it has most likely cleaned up and is back to normal again have the codes erased hopefully thats the end of it and you wont need a new cat. Best of luck ''
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Monday, May 16th, 2011 AT 1:29 AM
Tiny
WRENCHTECH
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You got to me kidding with that.
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Monday, May 16th, 2011 AT 1:33 AM
Tiny
MECHTRIX
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Oops" inefficiently
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Monday, May 16th, 2011 AT 1:35 AM

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