codes 41 and 91

Don't just skip this because it tells you something you may already know. Get out your voltmeter or multimeter and check the O2 sensors. Look for vacuum leaks while you are at it. Old vacuum hoses are suspect, and may be the cause of a vacuum leak.

Code 41 or 91 - O2 indicates system lean. Look for a vacuum leak or failing O2 sensor.

The computer sees a lean mixture signal coming from the O2 sensors and tries to compensate by adding more fuel.

The following is a Quote from Charles O. Probst, Ford fuel Injection & Electronic Engine control:
"When the mixture is lean, the exhaust gas has oxygen, about the same amount as the ambient air. So the sensor will generate less than 400 Millivolts. Remember lean = less voltage.

When the mixture is rich, there's less oxygen in the exhaust than in the ambient air , so voltage is generated between the two sides of the tip. The voltage is greater than 600 millivolts. Remember rich = more voltage.

Here's a tip: the newer the sensor, the more the voltage changes, swinging from as low as 0.1 volt to as much as 0.9 volt. As an oxygen sensor ages, the voltage changes get smaller and slower - the voltage change lags behind the change in exhaust gas oxygen.

Because the oxygen sensor generates its own voltage, never apply voltage and never measure resistance of the sensor circuit. To measure voltage signals, use an analog voltmeter with a high input impedance, at least 10 megohms. Remember, a digital voltmeter will average a changing voltage." End Quote

Measuring the O2 sensor voltage at the computer will give you a good idea of how well they are working. You'll have to pull the passenger side kick panel off to gain access to the computer connector. Remove the plastic wiring cover to get to the back side of the wiring. Use a safety pin or paper clip to probe the connections from the rear. The computer pins are 29 (LH O2 with a dark green/pink wire) and 43 (RH O2 with a dark blue/pink wire). Use the ground next to the computer to ground the voltmeter.

Most of the common multimeters have a resistance scale. Be sure the O2 sensors are disconnected and measure the resistance from the O2 sensor body harness to the pins on the computer.

Note that all resistance tests must be done with power off. Measuring resistance with a circuit powered on will give false readings and possibly damage the meter. Do not attempt to measure the resistance of the O2 sensors, it may damage them.

The O2 sensor ground (orange wire with a ring terminal on it) is in the wiring harness for the fuel injection wiring. I grounded mine to one of the intake manifold bolts

Do you have one of those chrome K&N filter oil fill caps on the valve cover instead of the factory one?

**edit: were the 41 & 91 codes key on engine running codes or continuous memory codes? If they were key on engine running, they're "always lean" codes. If they're continuous memory codes, they're "o2 sensors not switching" codes.

I got rid of my 41/91 KOER codes finally and they have been gone for weeks now - I checked my timing, EGR, PCV system, vacuum lines, checked for vacuum leaks with starter fluid - then propane, carb cleaner and so forth - never found any leaks anywhere, intake gaskets, oxygen sensors, plugs, wires, wiring, connectors, salt/pepper shakers, etc.... and everything you did and then some. I have an adjustable FPR and the only way I could get the codes to go away was to bump the fuel pressure 5lbs higher. Interesting thing now is with timing at 14 degrees and a 190 degree thermostat the car runs COOLER with higher fuel pressure - noticably so! I guess this means I was really running lean after all since the car runs perfectly and I am getting low 20's mpg (23 or so) under normal driving and 20 mpg with some aggressive use.