I have tahoe 08. 5.3l

Still have both code !

Just clean purge solenoid valve


Any help !

Possible causes
- Missing or loose fuel cap
- Incorrect fuel filler cap used
- Fuel filler cap remains open or fails to close
- Foreign matter caught in fuel filler cap
- EVAP canister or fuel tank leaks
- EVAP system hose leaking
- Fuel tank leaking

any big leak can cause p0455. if you have already looked at gas cap and the common things its frequently the vent valve is full of junk.

420 is supposed to be the cat is bad. through on a vehicle this new thats not normal. i would check the connections and stuff first

Ammm ..

Car is now 190k miles

But everything is ok and the car response is fast .

So better start with what ?

in that case it may be. it is supposed to have other codes for bad sensors. i say supposed to because its not 100 percent perfect system, it really just means the rear o2 is going into that range compared to the front, sometimes its the cat sometimes it needs sensors

what have you done to check the evap?

I clean the purge vent solenoild the near to tank . And still same .

Now i want to replace the cap .

Evap system cause the cat problem ?

not likely related. did you try activating the vent valve with a battery, could be stuck? vent valve is in the back by the tank, purge valve is on the engine.

No i don check it .. I know it is near under tank . I just clean it .

Don know how to solve the problem

cleaning it wont do anything if its stuck, you have to try and actuate it with a battery and some jumper wires.

P0420 bank1 convertor low effienciency. Check for exhaust leaks. No engine misfires present or any other codes present? I'd replace the convertor. Honestly it'* not that uncommon.
P0455 evap system has a large leak. Check for power at the vent valve connector. If a scanner is handy command the valve open and close with a test light to verify the circuit is good. Have also seen fuel tank pressure sensors flake out causing a bad reading, making the Ecm think there is a leak. Best overall test is having the system smoked to check for leaks. Also check the fuel cap seal. Any small cracks will cause a leak.

Thanx all .. Today i will replace the cap ,,

Before i have a random misfire with p0420 and 455 and make clear after change sparks plug + wires ,, now only both codes