I have a 99' A4 1.8T Fwd and I'm pulling P0455, P0422. I've checked the gas cap so i don't think its that. I don't want to spend the money and have the shop diagnose it. Help?

 

p0455 is evaporative emision control system leak detected,

and p0422 is Main Catalyst Efficiency Below Threshold (Bank 1).

i dont know if this will help. the first code i would check the lines running to your evap canister and the second code sounds possibly like an o2 sensor.

 

^^^ yup what he said...
But if u have a test pipe or high flow cat then u might just need to do the defouler mod...
If u don't then its prolly, like stated above an o2 sensor...