Just because you changed the exhaust doesnt mean you have no warranty. They can not tell you that unless they can prove that is what caused it and then have to state it in writing. I have headers on my tundra with the stock 02 sensors and my warranty is just as good now as the day i bought the truck.
Read this, very interesting..
http://www.tundrasolutions.com/forums/questions-and-comments/59196-aftermarket-parts-warranty/
Back to your question.....= An oxygen sensor is an electronic device that measures the proportion of oxygen in the gas(exhaust) being analyzed. The sensing element is usually made with a zirconium ceramic bulb coated on both sides with a thin layer of platinum and comes in both heated and unheated forms. The most common application is to measure the performance of internal combustion engines in automobiles and other vehicles. Divers also use a similar device to measure the level of oxygen they are breathing.
Indicates the level unburned fuel in the exhaust so the computer can adjust the air- fuel level.
Yes, it can cause the check engine light to come on, but so can about 106 other things.
What code is in the ECU???? Its easier to help you if we know what code has been stored. This will point us in the correct direction. Just changeing the exhaust should not set an engine light, unless you didnt plug the sensor back in, or they are in a different position than when the vehicle was stock.
Let us know the mods done to your vehicle, this may help to decifer the engine light problem.
If you dont have the code, most autoparts stores will read the codes for free, then you can post it here, so we can try to assist you better.
Thanks