I change out -all- the relays in the relay box fairly often, they are pretty inexpensive and i see no reason to pull out my hair looking for 1 of 6 relays that costs less than 20 minutes of my time to a customer. The the plug for the ecm that is right under the battery has a tendency to get battery acid on it and corrosion in it, and sometimes the wires right at the connection have broken, and the wires in the curve of the harness, where the throttle body harness is part of the main engine harness get pulled to where they break a 'wire weld' where the wires were welded together during manufacturing of the wire harness.