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420 code with a check engine light - 2005 Buick LaCrosse

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Code 420 means the O2 sensors are telling the computer the catalytic converter on bank one is not working correctly. Bank one is on the side with cylinder one.
The problem could be the cat, or one of the sensors, or a fuel mixture problem such as a vac leak in the engine or injector problem.
The sensors would need to be tested.

Posted on Jan 21, 2013

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get it checked by code reader when they flash shouldnt drive it may cause more damage

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I have a 2003 saturn ion 4 door, it has a ton of miles 194k. Last week the alternator went out do I replaced it without a problem, then the day after the check engine light came on I hook up the diagnostic...


p0420

no FSM> and not looking up P0420?
no engine stated, so ill get it wrong too..?????????????
420 is cat monitor failure. "efficiency"
the most hard of all.

ill make a list from hard and expensive .
1: bad sensor 2 rear. you failed to do both, that was WRONG.
2: bad cat. (i can weld on any cat for $99 bucks.) i do it last.. dead last.
3: engine running real bad. BUY a scan tool, now.

so the o2 is new that leaves #2 or #3.
i never shot gun cats. but shops do, its not their cash, huh?

i connect a real scan tool (not one that misses factory DTC codes)
then look at pendings for P042o , see funny sensor readings there, bingo.
i check every PID. for all sensors to be sure they READ right.
ECT AT 180F+ AND HOLDING? HOT.>?,
is the engine holding closed loop status at idle, no fix that NOW.
is LTFT (long term fuel trim ) near 0 or at - 40 or + 40% (fix that next)
if trim is good, the engine.
If the front 02 is new, the AFR will be correct and the the front 02 will swing fast and the trim will be near 0 , -4 is near best.
tad rich , natural.
once the engine is running Stoich at light flat cruise. (close loop) the
cat will heal its self (bake dry) and stop throwing 420. ( a hope deal)
if not the cat is bad.

ive cured some of the most hard 420s,.

on some ratty engines, a new cat can in fact hide it , until it cooks to death and next year 420 are back.
id never do that because gas is expensive. fix engine first.

Jun 24, 2014 | Cars & Trucks

1 Answer

Check engine light on the check engine light code isP0420


420 is catalyst below efficiency. On rare occasions it can be the post O2 sensor/s. Sorry to say, it is the catalytic converters. They are not cheap on your car.

Sep 05, 2013 | Toyota Cars & Trucks

1 Answer

Engine Light on code PO 420


You Prabably need a new catalytic converter. Very common problem with toyotas. You will need to get a dealer part as the aftermarket converters will put your check engine light on eventually.

Dec 20, 2010 | 2000 Toyota Corolla

1 Answer

420 code catalyst system efficiency below threshold (bank 1). EGR replaced along with both upstream and downstream 02 sensors on bank 1. Check engine light keeps coming on.


You could have an enefficient converter. Question is what caused it to go bad? I have attended seminars on code 420, Possible causes are lack of spark or clogged fuel injectors. there have been times where the spark plug wire lays on the engine and over time it wears a weak spot in the side of the wire, its not enough to throw a miss fire code but it keeps the full amount of spark from getting to the plug and not burning the all the fuel in the cyl. A leaky injector would also cause to much fuel to be deliverd into the cyl and not get burned.

Hope this helps.

Jan 20, 2010 | Chrysler 300C Cars & Trucks

1 Answer

I have 1999 chevy cavalier that has an engine code of 420


Code 420 is a catalytic converter is not working properly.Check the converter for leaks and the rear oxygen sensor for leaks.If they are good then you should replace the converter and clear the code.Good luck.Good luck.

Nov 18, 2009 | 2000 Chevrolet Cavalier

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