ok guys, i just recently replaced an 02 sensor. Front right. The other sensors are less than a year old as well. I replaced it because it was throwing some different codes and I replaced all plugs and relocated the o2 plug because of oil getting in it as well.

So after getting all that done the rover was fine for about a week or so. I noticed gas milage was still threw the roof (arount 10mpg or so). Then the CEL pops on and Im getting P0455. I check the gas cap and it "seems" fine, but I know you guys will tell me to replace it, which I will. If for some reason the gas cap is not the culprit, are there any other things I can check myself before having a smoke test done (thats what I see people doing in other threads, a smoke test).

regards,

 

Bad purge valve.

 

Not necessarily either the cap or purge valve. The 455 is a gross leak that can be caused by a number of issues, mainly cracked lines and has nothing to do with poor mileage.
You may want to replace the cap, they do fail some times, but I would suggest a smoke test to find the leak.

 

I had the P0445 off and on sporadically. The purge valve had already been replaced and it still came and went erratically. Turns out the last owner had the fuel delivery module replaced and they torqued it down so hard they pushed the gasket sealing the top of the fuel tank right into the tank! Once it was put back on properly no more leak or P0445! This may be a rare scenario, but I thought I would toss it out there.

 

I had the 0445 and a (0440?) code in my 03 a while back.

Try checking the tubes joining the carbon canister. If they are pressed too far into the rubber boots the tube will block and restrict air flow. I adjusted mine and installed zip ties to keep them put and the 0445 code cleared immediately.

Then I replaced the purge valve and lost the (0440?) code. If you go that route consider doing a search for the Hyundai substitute part. I saved a bit of money and Auto zone had the part in house.

(Can't remember of it was 0440... could be wrong)