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Trouble code P0420

M

marx3

New member
Joined
Aug 21, 2011
Location
Denmark
1999 V70, B5244S2
I keep getting this P0420 code with the engine light coming on too.
Engine runs great, fuel mileage is as expected, power feels fine.

I just replaced the O2 sensor in front of the cat.
No difference.

Since the car’s overall performance is good, I can’t see the cat being clogged.

What else can trigger this code ?
 
Yep. I do them all the time and they look fine Oil/coolant consumption can kill em. Misfires kill em fast too. Basically anything that contaminates or overheats it.
 
dammit. Well, at least it's pretty easy to replace.
alschnertz did you feel an increase in performance after replacing yours? or did you just get rid of the check engine light?
 
I did not notice any change in performance.

I did notice that my winter gas mileage (because we're required to use "winter-blend" fuel from Oct. to April) went up from about 28.5 mpg to around 30.0 mpg. But that's such an insignificant increase that I'd be reluctant to say it was due to the cat. replacement.
 
I'd put fresh high-quality O2 sensors on while you are in there. It may help with the fuel economy.

I believe a cracked exhaust manifold can cause a cat to fail. Extra air can cause it to get extra hot. Maybe clean/replace injectors and MAF too. (may be a bit excessive, but it can't hurt.)


I just did the cat in one of my cars for a 420 code and it had a cracked manifold too. I've replaced cats and cracked manifolds on more than a couple other occasions.

A coworker claimed he had a car with repeat 420 codes and cat replacements and that an O2 sensor was the final fix, but I have never seen this, just heard claims. I usually just recommend the 02 sensors for good measure.
 
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Most of the cats going bad are not plugged or restricted at all. May have just ran rich for too long, contaminated by coolant or oil burning. P0420 99% of the time is a cat needing replacement.
 
One other thing I've noticed is the $100 Magnaflow universal cats I use on my Toyota Matrix only last 3-4 years before going bad, I assume it's a matter of you get what you pay for.
 
update: I swung by an inspection place and the guy checked emmissions. All good. Lambda, CO etc completely perfect.
Even though the check engine light was on.
So It seems the cat and the O2 sensor are both working as they should
The check engine light comes on after about 50 miles of driving. This time with NO trouble code stored. What the heck...

Could this be the 2. bank O2 sensor causing this? that's the one that lights the check engine light, as I understand.
 
The code can mean that the catalytic converter is not meeting the specs it was designed for. It doesn’t mean that it won’t pass emissions testing. It can mean that the performance of the catalytic converter has dropped off to the point it will trigger a code to tell you it’s time to replace the catalytic converter.
 
If you get the light on with no codes shown you may want to try using a different code reader. Some inexpensive generic readers can't display factory codes, may have problems reading some of the protocols, etc.
 
When you did the emissions check did you look at the NOX reading and compare it to allowable limits? If your car was close to the limit but passed then it's the cat efficiency is going down.
 
I dont remember the exact values, but I could swing nby that guy againg and watch the NoX this time.
But you are probably all right... the cat is going south. I just keepo hoping to find some easier cause of the problem :-)
 
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