I don't have a reference for those codes. '96 and newer models use three-digit fault codes that get a lot more specific. In this case all you know is there's a problem with the fuel / air mixture, but you don't know why. If it's too lean, you'll have hesitations and stumbling on acceleration. If it's too rich, the engine will run fine but the extra fuel will just go out the tail pipe.
With non-descriptive codes like these, your mechanic needs to start with viewing live data on a scanner. That will show if the oxygen sensor is staying too rich or too lean too long. That sensor can only detect unburned oxygen, not fuel. Under normal operation, the exhaust gas switches from too rich to too lean one or two times per second, with the average being correct. Most commonly the exhaust will be too lean, and the computer will command more fuel to correct that condition. There's four places to look for a lean condition. The most common is a spark-related misfire. With old spark plugs or wires, when a cylinder misfires, the unburned fuel and oxygen go into the exhaust where the oxygen is detected as a lean condition. The Computer doesn't know that only one cylinder is responsible for that so it commands more fuel from all of the injectors. You'll smell raw fuel at the tail pipe, but the oxygen sensor will continue to report a lean condition no matter how much fuel is added. Based on oxygen sensor readings, the Engine Computer can modify fuel metering calculations by about plus or minus ten percent.
The next thing to consider is low fuel pressure. That is almost always accompanied by other problems. There's two forces that determine how much fuel sprays from an injector when it is pulsed open. Those are fuel pressure and intake manifold vacuum. When that relationship changes, it shows up in the exhaust and the computer corrects for that by adjusting the amount of time it holds the injectors open.
The last two things involve the air entering the engine. All of that air has to go through the mass air flow sensor. The computer calculates the correct amount of fuel based on the weight of that air. If there is a vacuum leak, the computer won't know about that air. The same happens when there's a leak in the fresh air tube between the mass air flow sensor and the throttle body.
In all of these cases it's a lean condition that results in too much fuel entering the engine. You can also look at the short and long-term fuel trim numbers on the scanner. If they are high positive, the computer is adding fuel to the metering calculations above and beyond what was programmed in at the factory as a starting point. If you find high negative numbers, the computer is trying to reduce the amount of fuel, and in this case, without success. That would indicate the need to look for the cause of running rich. Fuel pressure that's too high is one possible cause, but a leaking injector will allow too much fuel also. This is where a good engine performance specialist will look at the readings from all of the other sensors to figure out why the computer thinks more fuel is needed.
Tuesday, January 14th, 2014 AT 8:49 PM