I have a 2003 Tahoe LS 4wd 5.3 v8 flex fuel engine with 100,000 miles The truck is stock the only aftermarket part is a K & N air filter. I have a slight mis at idle that does NOT go away I have changed spark plugs & wires. fuel filter & had the intake gasket replaced & recently had the number 6 fuel injector replaced (it shorted out)...& just now the check engine light came on the code was PO420.... Cat converter but the truck runs well (except for the slight mis at idle) it still has power so I do not think the converter is cloged/blocked....Thanks in advance for any help

I have a 2003 Tahoe LS 4wd 5.3 v8 flex fuel engine with 100,000 miles The truck is stock the only aftermarket part is a K & N air filter. I have a slight mis at idle that does NOT go away I have changed spark plugs & wires. fuel filter & had the intake gasket replaced & recently had the number 6 fuel injector replaced (it shorted out)...& just now the check engine light came on the code was PO420.... Cat converter but the truck runs well (except for the slight mis at idle) it still has power so I do not think the converter is cloged/blocked....Thanks in advance for any help

PO 420 is the driver side cat is defective....to check for this when the engine is cold start,,, then go under vehicle.. with a hand on each cat ....if the passenger side cat gets hot first, and the driver side cat heats up slower you know the cat is bad...also the intake gasket problem may have caused this...

since the vehicle has 100,ooomi it is time to replace the O2 sensors...this is the life expectancy....

not replacing the upstream O2 sensors will damage the cat converters...

bad/poor idle is usually vacuum leaks,,,,or the IAC/throttle body needs cleaning with throttle body cleaner and a tooth brush to gently remove deposits....the IAC must not be forcefully cleaned just gently brush off the deposits with the cleaner aiding this..

since the air ducting must be removed clean the MAF sensor wires...use CRC MAF cleaner,,,just spray cleaner on the small sensor wires do not touch wires....let dry then reinstall...

good luck and let the forums know how it went...

Thank you for the advice/info will replace o2's & I will clean the TB & MAF after work today & I will post an update

PO 420 is the driver side cat is defective....to check for this when the engine is cold start,,, then go under vehicle.. with a hand on each cat ....if the passenger side cat gets hot first, and the driver side cat heats up slower you know the cat is bad...also the intake gasket problem may have caused this...

since the vehicle has 100,ooomi it is time to replace the O2 sensors...this is the life expectancy....

not replacing the upstream O2 sensors will damage the cat converters...

bad/poor idle is usually vacuum leaks,,,,or the IAC/throttle body needs cleaning with throttle body cleaner and a tooth brush to gently remove deposits....the IAC must not be forcefully cleaned just gently brush off the deposits with the cleaner aiding this..

since the air ducting must be removed clean the MAF sensor wires...use CRC MAF cleaner,,,just spray cleaner on the small sensor wires do not touch wires....let dry then reinstall...

good luck and let the forums know how it went...

I did clean the mas air & throtle body & it helped alot the miss is now gone thank you very much. I could not get the o2 sensors yet I have to order them. I will check the cats as you said but that will have to wait a day or 2 because of bad weather here & I have no garage to use ..