My wife's car was backfiring occasionally with NO check engine light coming on. I replaced the fuel filter and that seemed to help quite a bit. We drove the car to Vegas from Colorado and on the way back the check engine light comes on but is driving fine. I take it to Checker and it is a # 2 cylinder misfire. I replace all the plugs. Problem goes away even though they looked fine. Another few days it comes back. Knock Sensor. Guy says probably just moved a sensor in process of changing plugs. Resets computer. Another week or so, Check Engine Light again, this time engine shaking rough at idle. Take it to Autozone. Misfire on # 2. They also give me a readout saying it is either1. Ignition (plugs, wires, coil) 2. Vacuum leak 3. Fuel Injector 4. Low or high fuel pressure. Well, can't be plugs (just changed them). To check the coil I swap the front one (#2) with the middle one thinking if it comes up # 2 again,not the coil. Wiggle somes wires in the process and reassemble everything. Wow, problem goes away and so far has not come back. I don't know what the problem was in the first place as the check engine light not coming on and car driving great. Any ideas???