Corolla 93-97, 1.6L, How to Located ECU and How to get the CODES off of it?

[Naxxal]

Hello everyone,
I have Corolla 94, 1.6L engine, I have modified my car and replace the whole piping, starting with headers, O2 sensor, Cat, and to Muffler. Well i used to get 25mpg high/city together with 87 Onctane gas, no i get 21 mpg high/city but only with 91-93 Octane grade!!! get only 18 mpg with 87 Octane. By the way, the engine light comes on every time i get to 55 mph and stays on, does it every time i restart the car. MY questions are,

1. Where to i locate the ECU in my car?
2. How do i get the Codes off of ECU?
3. What code numbers need to be there?
4. Well, all thi infomration on how to check my ECU, if its function properlly or not?

Thanks

[veedubmechanic]

ecm is behind ash tray pannel. Did you remove any 02 sensors? it needs 2 o2 sensors, one infront of cat and one behind. Also aftermarket cat or test pipe could cause this.