Why can't I fix my codes?

Were codes cleared after you installed new parts?

Knowing nothing else of the symptoms, I'd clean the IAC if it was not a new one which was installed. If it was an aftermarket version, ensure it was installed correctly (some install upside down).

As a separate issue, check your ECT's calibration. If out of spec, that can create the ECT and fuel-trim codes. The sensor itself could be fine but you might have a wiring issue to or from the sensor.

Good luck.

Check the wiring and the sensor resistance...

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 :

50 degrees F = 3.52 v
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
212 degrees F = .47 v
230 degrees F = .36 v
248 degrees F = .28 v

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
212 degrees F = 2.07 K ohms
230 degrees F = 1.55 K ohms
248 degrees F = 1.18 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/

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

Code 41 or 91 - O2 indicates system lean. Look for a vacuum leak or failing O2 sensor. Code 41 is a RH side
sensor, code 91 is the LH side 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.
The O2 sensor voltage should switch between .2-.9 volt at idle.

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.

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. The computer pins are 29 (LH O2 with a dark green/pink wire)
and 43 (RH O2 with a dark blue/pink wire).

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.

If you get only code 41 and have changed the sensor, look for vacuum leaks. This is especially true if you are having idle problems.
The small plastic tubing is very brittle after many years of the heating it receives. Replace the tubing and check the PVC
and the hoses connected to it.
A secondary problem with only a code 41 is for cars with an intact smog pump and cats. If the tube on the back of the heads
clogs up the driver’s side, all the air from the smog pump gets dumped into one side. This excess air upsets the O2 sensor
calibration and can set a false code 41. The cure is to remove the crossover tube and thoroughly clean the insides to that
there is no carbon blocking the free flow of air to both heads.

Have you checked the O2 sensor voltage and the ECT readings?

i am having the same problems i have a 331 gt40's ssi intake,smog delete and no cats, i was having an idle issue and i put a iac throttle plate form ford, it fixed the idle for a little bit then all of a sudden the idle went retarded and now i have the lean o2 codes, but my
o2s are switching fine and i am running pig rich, maf reads fine same with the act and ect,are all working fine at idle and while driving
i was going to have the car dyno tune but i wanted to get this fixed before i get it tuned