P0141 Code 2011 Hyundai Accent Hatch

Question

Hey Scotty, thanks for everything you do! Hoping you can give some advice to me. I've got a 2011 Hyundai accent with 296,000 km on it. Right now I'm very rich. Especially on a day below 20 C, 68 F. On a day like that I'm about 9 litres per 100 km, 26 mpg. This only started happening this winter after I got a P0138 code for my downstream O2 sensor having high voltage. I replaced it with an NGK O2 sensor from Canadian Tire and it didn't seem to fix the issue. Rather than the P0138 code though, I got a P0141 code for my sensor heater. My engine light did shut off about two weeks back, but still getting that terrible mileage on cooler days. So thinking it could still be the downstream O2 heater? On very hot days my car has great mpg. Back to what Hyundai says it should be around, 36.2 mpg.
- spark plugs have been replaced with platinum NGK.
-ignition coils have not
-mechanic said they couldn't find any exhaust leaks
-i haven't gotten any codes for my injectors, cat, or anything like that
- I keep up on high mileage conventional oil changes and fuel system cleaner... Was using Gumout Regane High Mileage and now use Purple Royal Max Clean
-i just use base fuel from Esso
- I also cleaned my MAP sensor with sensor cleaner
-my breaks aren't rubbing or stuck anywhere
-I doubt it would affect anything, but I do have two oil leaks that are very very slow. One from my pan and one from top of valve cover gasket. Oil is just slowly oozing out from them, but my oil level isn't going down. I use high mileage conventional oil.
Keep my cabin and ail filters clean and replace when needed.
-checked my fuses and they all looked good
Could the new sensor be faulty? Or perhaps could I be a wiring issue leading to the sensor?

Any help on all of this would be much appreciated!!!

scottykilmer · Answer

Well one you can only use original equipment oxygen sensors. It's very very critical to the computer system. I would start with a factory sensor and see what happens. Because it's so complex. If you have a part that doesn't work right and you start hunting you pull your hair out and never fix it in the first place