Any previous history to share? How many miles and how long since the spark plugs were replaced?
Anything that causes a misfire will result in unburned fuel in the exhaust. That will be burned in the catalytic converter, however, before it gets there, the oxygen sensor will detect the unburned oxygen in that exhaust and will report to the Engine Computer that the engine is running too lean. The computer will command more fuel to all of the cylinders on that side of the engine. No matter how much extra fuel it requests, there will still be that unburned oxygen that gets detected. O2 sensors don't detect unburned fuel, just unburned oxygen.
Backfiring can be caused by bad spark plugs, ignition coils, plug wires on older engines, and a cracked core in the crankshaft position sensor. The air gap of the crank sensor is critical too. If it is installed incorrectly, it can cause intermittent stalling, intermittent misfires, and backfiring.
When there is a misfire, the Engine Computer will detect it and set the appropriate diagnostic fault code in memory. The Check Engine light should be on too. Most auto parts stores will read those codes for you for free.
Saturday, January 22nd, 2011 AT 6:04 PM