Code 12,21,41,91, now 12,42,92

jrichker said:

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 42 & 92 (running) System rich - Fuel control or (memory) System was rich for 15 seconds or more (no HO2S switching) - Fuel control. Look for leaking injectors, fuel pressure too high, cylinder(s) not firing due to bad ignition.

Since the O2 sensors were the last thing you replaced, they may be defective. The parts guy may have given you the wrong ones, or the new ones are bad out of the box.

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xpeteyjtx said:

Go HERE for a list of what the codes mean.

Are you getting the codes with the Engine Off or On? Pull the codes with both the Engine On and Off, so that you have a better idea of what is going on. Have you cleaned your TB (code 12) like jrichker suggested? It seems like you need to check some grounds and connections, and then possibly, replace some sensors. Whenever I mess with connections, I always apply dielectrical grease to the pins/connectors... it helps prevent corrosion and stuff like that. Good luck.

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jrichker said:

The code 64 means that either you have wiring problems
or the ACT sensor element has shorted to the metal case it
is mounted in. Remove the ACT sensor and check the
resistance between the metal threads and the electrical
contacts. You should see more that 1 megohm (1 million
ohms). If the sensor is OK, then you have wiring problems.

Diagram courtesy of Tmoss & Stang&2birds

Remove the passenger side kick panel and disconnect the
computer connector by loosening the 10 MM bolt. If you
try to do the next tests without disconnecting the
computer connector, yo will get false readings.

Then check the resistance between ground and the
lt green/pink wire on the ACT socket. You should see more
that 1 megohm (1 million ohms). Be careful and don't
touch the metal part of the meter leads or you'll get a false
reading. Then measure the end to end resistance of the
lt green/pink wire on the ACT socket and pin 25 on the
computer connector. You should see less than 1.5 ohms.

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/88-91eecPinout.gif

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jrichker said:

Vacuum leak or a bad electical connection could have casued a code 41/91. Both oth those could have been fixed when you removed & reinstalled the injectors.

Code 34 Or 334 - EGR voltage above closed limit - Failed sensor, carbon between EGR pintle valve and seat holding the valve off its seat. Remove the EGR valve and clean it with carbon remover. Prior to re-installing see if you can blow air through the flange side of the EGR by mouth. If it leaks, there is carbon stuck on the pintle valve seat, replace the EGR valve ($85-$95).

If the blow by test passes, and you have replaced the sensor, then you have electrical ground problems. Check the resistance between the black/white wire on the MAP/BARO sensor and then the black/white wire on the EGR and the same wire on the TPS. It should be less than 1 ohm. Next check the resistance between the black/white wire and the negative battery post. It should be less than 1 ohm.

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.

Let’s put on our Inspector Gadget propeller head beanies and think about how this works:
The EGR sensor is a variable resistor with ground on one leg and Vref (5 volts) on the other. Its’ resistance ranges from 4000 to 5500 Ohms measured between Vref & ground, depending on the sensor. The center connection of the variable resistor is the slider that moves in response to the amount of vacuum applied. The slider has some minimum value of resistance greater than 100 ohms so that the computer always sees a voltage present at its’ input. If the value was 0 ohms, there would be no voltage output. Then the computer would not be able to distinguish between a properly functioning sensor and one that had a broken wire or bad connection. The EGR I have in hand reads 700 Ohms between the slider (EPV) and ground (SIG RTN) at rest with no vacuum applied. The EGR valve or sensor may cause the voltage to be above closed limits due to the manufacturing tolerances that cause the EGR sensor to rest at a higher position than it should.

This will affect idle quality by diluting the intake air charge

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