Do not crank the engine, but cycle the ignition switch from " off" to " run" three times within 5 seconds, then sit back and count the flashes of the Check Engine light. You're looking for two-digit codes. One flash, a short pause, then two flashes is code 12, followed by a longer pause that will be the start of the next pair of digits. Code 12 just means the ignition switch was turned off. Not all vehicles bother to display it. Code 55 just means end of message. If you want to double-check or if you missed a code, cycle the ignition switch off and back to " run" once and the sequence will start over. Some will repeat the sequence over and over until you turn the ignition switch off or start the engine.
The codes will indicate the circuit or system with the problem, not the specific part.
You might also check for a sticking front brake caliper as the cause of low fuel mileage.