Where to start with these codes...?

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

Replace the O2 sensors in pairs if replacement is indicated. If one is weak or bad, the other one probably isn't far behind.


Code 87 – fuel pump primary circuit failure. The fuel pump lost power while the engine was running.
Check fuel pump relay, check inertia switch, wiring to/from inertia switch, red wire going to inertia
switch for +12volts. Check the other side of inertia switch for +12 volts.

To help troubleshoot the 87 code, follow this link for a wiring diagram for 89-93 cars
http://www.autozone.com/servlet/UiB..._us/0900823d/80/19/59/5a/0900823d8019595a.jsp

For 79-88 cars use, http://www.autozone.com/servlet/UiB..._us/0900823d/80/16/71/3c/0900823d8016713c.jsp


Code 95 - the following test path is for 86-90 model Mustangs.

The 95 code is because at one time or another, the fuel pump relay hiccupped and didn't provide
power the pump when the computer told it to run. Sometimes this is a one time thing, other times
it is a no run or runs poorly condition.

To help troubleshoot the 95 code, follow this link for a wiring diagram http://www.autozone.com/images/cds/gif/large/0900823d80167162.gif

Using the diagram, check the red/black wire from the fuel pump relay: you should see 12 volts or
so. If not, check the inertia switch: on a hatch it is on the driver’s side by the taillight. Look for a
black rubber plug that pops out: if you don't find it, then loosen up the plastic trim. Check for
voltage on both sides of the switch. If there is voltage on both sides, then check the Pink/black wire
on the fuel pump relay: it is the power feed to the fuel pump. No voltage there, check the
Orange/Lt blue wire, it is the power feed to the fuel pump relay & has a fuse link in it. If there is
good voltage there & at the Pink/black wire, swap the relay.

Some Mass Air conversions neglect to run the extra fuel pump wire, and they always have a 95
code. See http://www.stangnet.com/tech/maf/massairconversion.html for more
information on the Mass Air wiring conversion.

Diagram courtesy of Tmoss & Stang&2birds


See the following website for some help from Tmoss (diagram designer) & Stang&2Birds (website host) for help on 88-95 wiring http://www.veryuseful.com/mustang/tech/engine/ Everyone should bookmark this site.

Ignition switch wiring
http://www.veryuseful.com/mustang/tech/engine/images/IgnitionSwitchWiring.gif

Fuel, alternator, A/C and ignition wiring
http://www.veryuseful.com/mustang/tech/engine/images/fuel-alt-links-ign-ac.gif

Complete computer, actuator & sensor wiring diagram for 88-91 Mass Air Mustangs
http://www.veryuseful.com/mustang/tech/engine/images/88-91_5.0_EEC_Wiring_Diagram.gif

Vacuum diagram 89-93 Mustangs
http://www.veryuseful.com/mustang/tech/engine/images/mustangFoxFordVacuumDiagram.jpg