p0420 code and stalling


Asked by Mar 09, 2016 at 11:26 AM about the 2004 Toyota Celica GTS

Question type: Maintenance & Repair

When I first bought this car a year ago I had a light come on for p0171 for
lean condition, so this is what I have done.  I changed the o2 sensor before
the catalytic because this is what I was told to change, but then I decided to
google this code, so I changed the mass air flow sensor, hoses, cleaned
throttle body and changed out the spark plugs.  Now I erased the codes
drove it and now I am getting p0420, so then I changed both 02 sensors
erased the codes, drove it a bit still getting the p0420 code and I stall when
coming to a stop, it will idle to like 1500 rpm then drop back down to 1k rpm
or sometimes stalls.  So, I just noticed I bought copper spark plugs ngk
ones.  Which spark plugs are good for a 2004 toyota celica gts 1.8L?  
Should I get denso? or could it be something else causing my stalling?

4 Answers


Just about all the spark plugs are good as long as you get the ones called out for your vehicle.-P0420 TOYOTA - Catalyst System Efficiency Below Threshold Bank 1 Possible causes... - Three way catalyst converter Bank 1 - Exhaust tube - Intake air leaks - Fuel injectors may be faulty - Fuel injector leaks - Spark plugs may be faulty - Improper ignition timing - Engine Control Module (ECM)... 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.

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Best Answer

I had a leak a while back on a fuel injector cause of a broken o-ring, replaced the o-ring no leaks. Maybe my spark plugs are faulty that I just bought? I already replaced the o2 sensors and I noticed I am getting high long fuel trim, what could cause this? I appreciate any info you got for me.


It looks to me that the catalytic converters are worn out. They don't have to be plugged to be bad, or they are just starting to get plugged up. Could relate to the idle issue. Fuel is a possibility, just need to do a leak-down test to see if any injectors are at fault, as this would not throw a code. It looks like the 02 sensors are doing there job telling you something is wrong. Copper NJK plugs are good plugs. So I would go to an exhaust shop that can test cats. to see for sure this is the problem.


Ok I appreciate it, wil, find a shop and get it tested.

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