error codes 12,21,41,91 NEED HELP!

Code 12 -Idle Air Bypass motor not controlling idle properly (generally idle too low) -
IAB dirty or not working. Take it off and clean it thoroughly with throttle body cleaner.
Clean the electrical contacts with non flammable brake parts cleaner at the same time.

Code 21 – ECT sensor out of range. Broken or damaged wiring, bad ECT sensor.
Note that that if the outside air temp is below 50 degrees F that the test for the
ECT can be in error.

The ECT sensor has absolutely nothing to do with the temperature gauge. They are
different animals. The ECT sensor is normally located it the RH front of the engine in
the water feed tubes for the heater.

The ACT & ECT have the same thermistor, so the table values are the same

ACT & ECT test data:

Use Pin 46 on the computer for ground for both ECT & ACT to get most accurate
readings.

Pin 7 on the computer - ECT signal in. at 176 degrees F it should be .80 volts

Pin 25 on the computer - ACT signal in. at 50 degrees F it should be 3.5 volts. It is
a good number if the ACT is mounted in the inlet airbox. If it is mounted in the lower
intake manifold, the voltage readings will be lower because of the heat transfer.

Voltages may be measured across the ECT/ACT by probing the connector from
the rear. A pair of safety pins may be helpful in doing this. Use care in doing it
so that you don't damage the wiring or connector.

Here's the table :

68 degrees F = 3.02 v
86 degrees F = 2.62 v
104 degrees F = 2.16 v
122 degrees F = 1.72 v
140 degrees F = 1.35 v
158 degrees F = 1.04 v
176 degrees F = .80 v
194 degrees F = .61

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.

Ohms measures at the computer with the computer disconnected, or at the sensor
with the sensor disconnected.
50 degrees F = 58.75 K ohms
68 degrees F = 37.30 K ohms
86 degrees F = 27.27 K ohms
104 degrees F = 16.15 K ohms
122 degrees F = 10.97 K ohms
140 degrees F = 7.60 K ohms
158 degrees F = 5.37 K ohms
176 degrees F = 3.84 K ohms
194 degrees F = 2.80 K ohms

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/

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

http://www.veryuseful.com/mustang/tech/engine/images/fuel-alt-links-ign-ac.gif

http://www.veryuseful.com/mustang/tech/engine/images/88-91_5.0_EEC_Wiring_Diagram.gif


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

Testing the O2 sensors
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.

Testing the O2 sensor wiring harness
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

The Thermactor Air System (smog pump) has no effect on the O2 sensors. Running a
cat H pipe without the Thermactor Air System will cause the cats to prematurely fail.

maybe the ect sensor problem is causing improper fuel mixture, bringing the o2 codes

bills93notch said:

I guess my question is if im getting both banks of the o2 sensors lean is it possible both are burned out or is it most likely the ECT sensor? the temp at my house garage is 48 degrees inside is there any other tests to chech the o2 sensors short of replacing them also would a rich condition cause backfiring?

Click to expand...

Jrichker hit upon this in his above-post. Check the signals for amplitude and frequency.

There are myriad other variables that can lead to lean codes as well.

Good luck.

Bill stated:

ok good enough so let me make sure i understand unplug the o2 sensor from the factory plug under the car remove the ecu cover and with the key off measure the resistance with the proceedure listed above?[/

Click to expand...

Bill, below is the juicy part of JR's list. One point of emphasis - he outlines a couple of tests. The first test is to check the signal output of each O2. This is a voltage reading that oscillates between ~0 and 1 volt, and is the test I noted before that I'd do.

Then JR goes on to another test involving resistance, which is where you disconnect the component. I think you might have merged the two ideas, which would make for one really strange test. I tried to break the two thoughts up into two distinct quotes.

jrichker said:

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.

Click to expand...

jrichker said:

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.

Click to expand...

bills93notch said:

ok great hissin50 thanks ya i must have merged the two that would make for a strange test one finaly clarifying question to measure the voltage output do you do this with the engine running or key on engine off?

Click to expand...

Since the O2 sensor only measure the oxygen in the exhaust gas, the engine must be running.
Unless you have some other exhaust to measure...like farting in the pipe...