That's what your mechanic has to diagnose. The problem here is either code could be caused by something that caused the other code to set. Uhm, ... What I mean is you could have a code 304, cylinder four misfire due to a lean condition, or you could have a code 171 lean condition due to the excess unburned oxygen caused by a misfire.
You might start by looking for a vacuum leak. That could set the lean code. All cars except Chrysler products use a mass air flow sensor to measure the weight of the incoming air. If any air sneaks in that doesn't go through that sensor, the Engine Computer won't know about it and it won't include that in its fuel metering calculations. You won't get enough fuel and may experience a hesitation or stumble. That can cause a lean condition to be detected.
Your mechanic would approach this by watching the oxygen sensors' readings on a scanner, then introducing an artificial lean condition by removing a vacuum hose momentarily, then by introducing an artificial rich condition by removing the vacuum hose to the fuel pressure regulator, if your engine uses one, and plugging it, or by spraying in propane from a small bottle. He will watch the oxygen sensors to see if they respond properly.
By the way, be sure there are no air leaks in the fresh air tube between the mass air flow sensor and the throttle body.
For the misfire, you need fuel and air, spark, compression, and the proper timing for each event. Timing isn't likely to be an issue because that would affect all the cylinders equally. Low compression is a possibility at the mileage you listed. You won't feel it yet if it isn't real low, but that can cause enough of a misfire for the computer to detect. For fuel, you can switch the injectors between cylinder four and one other one, erase the fault code, then drive the car and see if a code sets for cylinder four again or the one you moved the injector to.
For spark, there's an even chance a new spark plug can cause a problem. The gap can be wrong. It could be the wrong type of spark plug. Or, as one corporate trainer used to say, "we not only sell you new parts, we sell them to you pre-broken".
We recently had someone here with a Toyota with the same fault codes and theirs turned out to be a loose new spark plug. During the intake stroke air got sucked in that resulted in extra unburned oxygen going into the exhaust where it got detected as the lean condition. He never felt the misfire but there was a code for that too.
Sunday, March 15th, 2015 AT 12:03 AM