Help? Low Octane Fuel Causing a Code (P0420)
 

*Fuel

I know this code is for the catalyst showing low efficiency but, I was wondering if anybody can explain why?

If I put in low octane Ill throw a code with in 10 miles.

If I put in high octane the light stays off and with no code history.

could it be an O2 sensor?
:icon_teeth:

Anybody? I was told to use 15-40 because of the higher mileage. 3000 miles now with no problems.

Sounds like it is time for a ring job.

I got the same code on my 98 Blazer right after I replaced all the hydrolic valve lifters (Thank God the cheap Advanced Auto supply ones worked great saved 40.00 each), I think it was the extra smoke it got upon starting up, With the lifters and manifold gasket now I don't hear the water rushing thru the dashboard heater core anymore everytime I accelerate. I bet my performance can be improved with a bigger catylitic converter and that is what the code says but it's a lot of money either way. With old oxyegen sensors replacing one when you have many can throw the readings off enough that the light stays on for almost a year till the new sensors gets as bad on reads like the rest of them.( At least that what seamed to be happening on my other Kia). I will have to try the high octain and change the oil again.
David French

Generally this is caused by either a bad O2 sensor or a catalytic converter that is starting to fail.
The code is thrown when the readings of the O2 sensors both before and after (or in the cat on some models) are reading essentially the same. Or if one isn't reading at all as I have seen that throw this code once before myself.

They could be reading this way either because both O2 sensors have gone bad or because there is a problem with the catalytic converter itself.

Another potential cause is an engine temperature sensor not operating correctly. Incorrect temperature readings tend to throw off the computers air fuel calculations and causes all kinds of interesting issues. Especially emission control codes being thrown.

A much less common cause is having your timing slightly retarded for some reason. The higher octane fuel would offset this a bit as it is slower to ignite.
This could be cause by a number of things and should really only be looked into if all of the above checks out perfectly so I won't go into it further.

Just out of curiosity. What is your make model and mileage.

It can help to properly diagnose things.
Also unless there is very very good cause, using the incorrect oil viscosity in your truck can have some very negative results. An emissions control light I would say is not a good cause to change to such a thick oil.
15w-40 would be a bad idea in winter time especially when your truck should be running 5w-30 I believe.

If you want to know exactly why it is bad look here http://www.motoroildealer.com/tech_a...ity/index.html

My truck did the same thing the other day , i just bought it , its a 97 LT blazer with 105k miles , no issues i put gas it in from BP 87 , and ten miles later i had a check engine light with a P0420 code , at the advice of another blazer guy i added a half can of seafoam and it went away in 50miles or so . Has me a bit puzzled , was it nothing or something coming ??

It came back so im looking into the actual cat 02 situation to see if i can figure out the factual problem and not guess at one or the other .. any suggestions with this ??

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