Codes P0420 and p0430
 

After trans rebuild I have been getting randomly. I have replaced both upstream O2 sensors and every other week I get the code on onside or the other.

Last thing I have done is clean the MAF sensor and air temp sensor. So far no issues. But if it comes back any ideas?

Oh if this does come back I have heard it can fail you on emission testing I am good for another 9 months.

Re: Codes P0420 and p0430
 

Does your truck emit black smoke on startup?

Re: Codes P0420 and p0430
 

No black smoke

Re: Codes P0420 and p0430
 

DTC P0420
Catalyst System Low Efficiency Bank 1

DTC P0430
Catalyst System Low Efficiency Bank 2

Check the post catalyst 02 sensors for wiring damage.

Check for exhaust leaks.

Things that can degrade the catalyst itself..

An engine misfire
High engine oil or high coolant consumption
Retarded spark timing
A weak or poor spark
A lean fuel mixture
A rich fuel mixture

Its hard to believe that both catalyst went bad at the same time, so check it over real good.

Re: Codes P0420 and p0430
 

More info.
All spark plugs were replaced two months ago along with cap and rotor. All the old plugs were in good shape.
No other code have been happening. Just one week p0420. I hook up scanner and watch back o2 they appear good so I clear it. Then next week I get p0430. same thing again

I am thinking of running GM Fuel System Treatment PLUS P/N 88861011
or seafoam.

Re: Codes P0420 and p0430
 

Try ET100 from NAPA it is a fuel tank additive and injector cleaner that will help with carbion build up and a clean engine will help converter efficiency. I was getting a p0420 code also in my truck tried this and cleared codes and no light or code in 1500 miles.

Re: Codes P0420 and p0430
 

If this started just after a trans removal/installation, check around the trans bell housing to see if they pinched some wires or unhooked somthing and didn't get it back together. I'm thinking that the harness for both banks originates on the pass side of the engine and then bank 2 runs down the pass side of the bell housing where it hits small protective enclosures where the O2 sensors plugs are and, I'm thinking they are attached the the stud/bolts that hold then trans to the engine. Same for bank I except on the driver's side.

Re: Codes P0420 and p0430
 

Think I got it fixed after replace down stream O2 sensor and still get code and having the cats checked I did this to fix it.

I installed two 18mm no foul spark plug holders. One set I drilled out to accept the O2 sensor then screwed that into the stock one. After that I installed back into the exhaust and have not had an issue with it for two weeks. When before it would happen again after a day. Part number 42009 at autozone in the HELP section.

Re: Codes P0420 and p0430
 

If this is what fixed it, you just masked the problem. Chances are one of the issues in Clinebarger's post are killing the cats.:

"Check for exhaust leaks.

Things that can degrade the catalyst itself..

An engine misfire
High engine oil or high coolant consumption
Retarded spark timing
A weak or poor spark
A lean fuel mixture
A rich fuel mixture "