Help with DTC's?

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. Many times the end result is an engine that runs pig rich and stinks of unburned 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

Code 96 – KOEO- Fuel pump monitor circuit shows no power - Fuel pump relay or battery power feed was open - Power / Fuel Pump Circuits. The fuel pump circuit lost power at one time or another.

Look for a failing fuel pump relay, bad connections or broken wiring. The fuel pump relay is located under the Mass Air Meter on Fox bodied stangs built after 91. On earlier model cars is under the passenger seat. On Mass Air Conversions, the signal lead that tells the computer that the fuel pump has power may not have been wired correctly. See http://www.stangnet.com/tech/maf/massairconversion.html

To help troubleshoot the 96 code for 91-93 models see http://www.autozone.com/images/cds/gif/large/0900823d80195960.gif

Look for power at the fuel pump - the fuel pump has a connector at the rear of the car with a pink/black wire and a black wire that goes to the fuel pump. The pink/black wire should be hot when the test connector is jumpered to the test position. . To trick the fuel pump into running, find the ECC test connector and jump the connector in the lower RH corner to ground. http://www.mustangworks.com/articles/electronics/eec-iv_codes.html for a description of the test connector. it. No voltage when jumpered, check the fuel pump relay and fuse links.

Power feed: Look for 12 volts at the pink/black wire (power source for fuel pump relay). No voltage or low voltage, bad fuse link, bad wiring, or connections. Remember that on 92 or later models the fuel pump relay is located under the Mass Air meter. Watch out for the WOT A/C control relay on these cars, as it is located in the same place and can easily be mistaken for the fuel pump relay.

Relay: Turn on the key and jumper the ECC test connector as previously described. Look for 12 volts at the dark green\yellow wire (relay controlled power for the fuel pump). No voltage there means that the relay has failed, or there is a broken wire in the relay control circuit.

Clear the 41 & 96 code by disconnecting the jumper while you are reading the codes. This will clear all codes in the computer, but will leave the adaptive settings intact. I would drive the car for a while andd see if the 96 code come back. If it doesn't, then don't worry about it.

OrLackThereof said:

O2 sensors are 2 months old, replaced both sides. System has been smog-checked and visually inspected for vacuum leaks several times. Engine makes 7 inches at the manifold (lots of cam overlap). The O2 ground is grounded on an intake manifold bolt like you said.

Any reason why it would read only one bank? Ive always gotten codes from both bank sensors every time I get a lean/rich.


And when you say disconnect the jumper....is that the single wire connector?

Click to expand...

Seven inches seems a bit low, even for a wild cam. A leak on the LH side only due to a shifted lower intake gasket could set the code for the LH side and not the RH side. A recient post from one of the guys here on Stangnet had some pics of his gasket that had shifted & created a vacuum leak that he couldn't find by any other means.

To clear the codes, disconnect the single wire connector from the jumper.

jrichker said:

Seven inches seems a bit low, even for a wild cam. A leak on the LH side only due to a shifted lower intake gasket could set the code for the LH side and not the RH side. A recient post from one of the guys here on Stangnet had some pics of his gasket that had shifted & created a vacuum leak that he couldn't find by any other means.

To clear the codes, disconnect the single wire connector from the jumper.

Click to expand...

I am not sure if it is who JR is thinking of, But Jason (Vristang) had a similar issue that fits what JR is saying (in case you want to read his thread to compare symptoms, etc).

Good luck.