For the past two months or so my Elantra has been getting horrendous gas mileage. On a good week it will get 175mpt. Normally, I would get 250+. I pulled codes and found that the 2nd oxygen sensor has some burns on the wires, and I have a code for the IAC. I don't normally drive the car, maybe 20 miles a week total. What can I do to get my gas mileage back up there. I already put new plugs and wires in when the BetaII engine went in. And I will be going back to stock Tiburon FX wheels and back to stock. Help!

You need to get the codes. You gave no useful information in this post. The IAC will be accompanied by other codes.

From my other unreplied to thread:

My never ending delima with CEL's on my Elantra is finally getting to me. This started a few months ago after I blew the engine and torque converter on the stock BetaI. After getting the BetaII engine and BetaI transmission back into the car, I have had these four codes. The P0131 and P0135 refer to the O2 sensor low voltage and heater circuit malfunction. I bought a new primary oxygen sensor from Advance Auto Parts, repolaced the old sensor, and the P0131 and P0135 codes still persist.

I thought the oxygen sensor may of been bad, so took it back to get a replacement. They gave me a new Bosch sensor, the same as the old one, put it in, and the same issue occured. I've been doing some research and it seems the ECU is very picky about replacement sensor's. I called around to local dealer's but the cheapest sensor I could find was 120USD. I really don't want to spend that kind of money just to get rid of of a oxygen sensor CEL. Would a sparkplug fouler be a cheap solution to this issue? I read up on them and it seems to make the sensor not get a direct reading on the air/fuel mixture.

Now for my other two codes, the P1513 and P1553. These two codes refer to idle air control issue. Before the engine was swapped out, I had no issues, but since I have the BetaII engine in, these codes still persist. I changed out the IAC, TPS, MAF to see if this would correct the issue, but it has not. I did some more digging around and believe my issue may be in the connector. I'm going to attempt to get a extra connector from a junked car to see if it solves the issue, if not, I will be back to the drawing board.

Doe's anyone have any experience/input into these CEL's? I'm really getting tired of these reoccuring issues. HELP!

IDK if this will help. Anyhow, here goes: cleaning the MAF, IAT and IAC and all the connectors possible with electrical contact cleaner. (although isn't the BetaII a MAP engine?)

About the O2 sensor, it needs to be a heated O2 sensor, or it will throw codes all day long. My 99 Elantra did that when I replaced the rear sensor with an unheated one initially. It was throwing those same 2 codes with the new unheated sensor until replacement with a heated sensor. A Bosch universal sensor is heated, I believe. I've had one of those as my primary for 2 years without problems.

I have the same problems with the Bosch O2 sensors. I bought the OEM Direct Replacement O2 sensor so I would not have to deal with wiring a connector. Some amount of miles later, same codes: P0131 and P0135. I put the stock O2 sensor back in (with 100,000 miles on it) and the CEL's turn off almost immediately (3 starts). I tried the universal Bosch O2 sensor, and same results. Seems that I can only use the stock O2 sensor for the rest of the cars life.