Really to take all the guesswork out, you need a code scanner to read what the trouble code is that is causing the check engine light. A common code is the P420 code which could be the O2 sensor or the catalytic converter. If you're over 100,000 miles and haven't done a tune up then it possible it's time for plugs and new coil packs. In which case the trouble code will tell you there is a misfire. But could a trouble code for the IAT out of range just be a dirty air filter and/or dirty MAF sensor. In almost any of these situations it couldn't hurt to change the air filter, clean the MAF, and run some Seafoam through your car.