Engine Code 12, 92; Code 12 will not go away!

What is your fuel pressure at? Does it fluctuate a lot? What are you pulling for vacuum?

Code 12 &412 -Idle Air Bypass motor not controlling idle properly (generally idle too low) - IAB dirty or not working. Clean the electrical contacts with non flammable brake parts cleaner at the same time.

IAC doesn't work: look for +12 volts at the IAC red wire. Then check for continuity between the white/lt blue wire and pin 21 on the computer. The IAC connector contacts will sometimes corrode and make the IAC not work. The red wire on the IAC is always hot with the engine in run mode. The computer provides a ground for the current for the IAC. It switches the ground on and off, making a square wave with a varying duty cycle. A normal square wave would be on for 50% of the time and off for 50% of the time. When the idle speed is low, the duty cycle increases more than 50% to open the IAC more. When the engine speed is high, it decreases the duty cycle to less than 50% to close the IAC. An old-fashioned dwell meter can be used to check the change: I haven’t tried it personally, but it should work. In theory, it should read ½ scale of whatever range you set it on with a 50% duty cycle. An Oscilloscope is even better if you can find someone who has one and will help.



Recommended procedure for cleaning the IAC/IAB:
Conventional cleaning methods like throttle body cleaner aren’t very effective. The best method is a soak type cleaner used for carburetors. If you are into fixing motorcycles, jet skis, snowmobiles or anything else with a small carburetor, you probably have used the one gallon soak cleaners like Gunk or Berryman. One of the two should be available at your local auto parts store for $22-$29. Take the solenoid off the body and set it aside: the carb cleaner will damage some types of plastic parts. Soak the metal body in the carb cleaner overnight. There is a basket to set the parts in while they are soaking. When you finish soaking overnight, twist the stem of the IAB/IAC that sticks out while the blocker valve is seated. This removes any leftover deposits from the blocker valve seat. Rinse the part off with water and blow it dry with compressed air. The IAC/IAB should seal up nicely now. Once it has dried, try blowing through the bottom hole and it should block the air flow. Reassemble and reinstall to check it out.

Gunk Dip type carb & parts soaker:



Setting the base idle speed:
First of all, the idle needs to be adjusted to where the speed is at or below 600 RPM with the IAC disconnected. If you have a wild cam, you may have to raise this figure 100-150 RPM or so. Then the electrical signal through the IAC can vary the airflow through it under computer control. Remember that the IAC can only add air to increase the base idle speed set by the mechanical adjustment. The 600 RPM base idle speed is what you have after the mechanical adjustment. The IAC increases that speed by supplying more air under computer control to raise the RPM’s to 650-725 RPM’s. This figure will increase if you have a wild cam, and may end up between 800-950 RPM

Remember that changing the mechanical idle speed adjustment changes the TPS setting too.

This isn't the method Ford uses, but it does work. Do not attempt to set the idle speed until you have fixed all the codes and are sure that there are no vacuum leaks.

Disconnect the battery negative terminal and turn the headlights on. Leave the battery negative terminal disconnected for 5 minutes or so. Then turn the headlights off and reconnect the battery. This erases the computer settings that may affect idle performance.

Warm the engine up to operating temperature, place the transmission in neutral, and set the parking brake. Turn off lights, A/C, all unnecessary electrical loads. Disconnect the IAC electrical connector. Remove the SPOUT plug. This will lock the ignition timing so that the computer won't change the spark advance, which changes the idle speed. Note the engine RPM: use the mechanical adjustment screw under the throttle body to raise or lower the RPM until you get the 600 RPM mark +/- 25 RPM. A wild cam may make it necessary to increase the 600 RPM figure to 700 RPM or possibly a little more to get a stable idle speed.
Changing the mechanical adjustment changes the TPS, so you will need to set it.

When you are satisfied with the results, turn off the engine, and re-install the SPOUT and reconnect the IAC. The engine should idle with the range of 650-750 RPM without the A/C on or extra electrical loads. A wild cam may make this figure somewhat higher.

An engine that whose idle speed cannot be set at 600 RPM with the IAC disconnected has mechanical problems. Vacuum leaks are the #1 suspect in this case. A vacuum gauge will help pinpoint both vacuum leaks and improperly adjusted valves. A sticking valve or one adjusted too tight will cause low vacuum and a 5"-8" sweep every time the bad cylinder comes up on compression stroke. An extreme cam can make the 600 RPM set point difficult to set. Contact your cam supplier or manufacturer to get information on idle speed and quality



Code 42 & 92 & 137 & 173 (engine 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.

Code 42 passenger side sensor, as viewed from the driver's seat
Code 92 is the driver side sensor, as viewed from the driver's seat.

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 87-93 5.0 Mustangs
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.


Backside view of the computer wiring connector:


87-90 5.0 Mustangs:
Computer pin 43 Dark blue/Lt green – LH O2 sensor
Computer pin 29 Dark Green/Pink – RH O2 sensor
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.

91-93 5.0 Mustangs:
Computer pin 43 Red/Black – LH O2 sensor
Computer pin 29 Gray/Lt blue – RH O2 sensor
The computer pins are 29 (LH O2 with a Gray/Lt blue wire) and 43 (RH O2 with a Red/Black wire). Use the ground next to the computer to ground the voltmeter. The O2 sensor voltage should switch between .2-.9 volt at idle.


Testing the O2 sensors 94-95 5.0 Mustangs
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 red/black wire) and 27 (RH O2 with a gray/lt blue wire). Use pin 32 (gray/red wire) to ground the voltmeter. The O2 sensor voltage should switch between .2-.9 volt at idle.



There is a fuse link for the O2 sensor heater power. According to Ranchero50, it is in the wiring near the passenger side hood hinge. Measuring the voltages will give a clue if it has shorted to the O2 sensor signal lead. The O2 sensor voltage should switch between .2-.9 volt at idle.

Oxford88Fox said:

Thank you for the reply, here’s an update from yesterday:

Ran fuel pressure test, checked out perfectly.

Ran codes again, code 12 was gone, however code 92 is still there... :/

I tested the harness for the IAC, and here is what I found:
KOEO at red wire: ~12.4v
KOER at red wire : ~14.2v

Car hesitates intermittently, and shifts hard (AOD)

I forgot to mention, I have done a complete tune up, new dizzy, spark plugs (motorcraft coppers), coil, and wires. Gaps are set at .054. 3G alternator with 4Ga wire installed as well, proper grounding too. New (NTK) oxygen sensors installed too.

Why would code 12 go away now, but 92 stay, and the car running pig rich, and misfiring intermittently?

Click to expand...

Read the first line of the Code 12 test path Idle Air Bypass motor not controlling idle properly (generally idle too low Evidently your idle speed problems have cleared up for a while...

Maybe this: "New (NTK) oxygen sensors installed". Factory replacements are Bosch.
You also said: "Vacuum is 15-17 at idle" . You could have a vacuum leak. Sine '88's are speed density, correct vacuum reading is critical. Might need a new BAP sensor.

Edit: sorry if you read before the edit. I didn't initially catch the fact that you'd replaced the O2 sensors. Did this problem exist before you replaced your sensors? Is it possible that you bought a bad O2 sensor? I don't know about NTK sensors, but I trust Bosch and motorcraft.

You report running pig rich, but the bucking/hesitation/falling on its face you're experiencing says otherwise. If you run 16:1 or higher, this is what happens. To get rich enough to get some spark blowout, you'd have to be in the 9:1 AFR range or lower, which I have a hard time believing. Your description is consistent with an O2 sensor telling the computer there's a rich condition, and the computer is leaning the car out to compensate. Except, it can't compensate, because the sensor is keeps reading rich under any circumstance. If you wanted to test under driving conditions, you could get an adjustable fuel pressure regulator and lower the pressure until you force the system to go lean. If you're really rich, it should run better and better as you turn it down, but I bet it won't. If you try this, drop 10psi with the vacuum off the regulator. The EECIV can adjust fuel by about 25% (12.5 on each side of stoich, so I've been told)

You could also diagnose the issue if you had some way to read air-fuel: reading plugs, a separate wideband, etc... However, if you don't already have a way, then new O2 sensors will be cheaper than the equipment. You can run Jrichker's O2 diagnostics, of course. However, if all you see is confirmation that their voltage is high, that won't tell you much. Hopefully, there will be something clear if you go down this path. Bosch sensors are $24 here: https://www.rockauto.com/en/catalog...10,exhaust+&+emission,oxygen+(o2)+sensor,5132

Don't pull the motor and rebuild unless you have properly diagnosed a mechanical issue. A brand new engine will not fix a problem with a sensor or the computer and you will be really upset if you put it all back together and it still runs like , especially if you make any mistakes because diagnosing compound issues is infinitely harder than 1 at a time.

I recently got a choppy reading from my O2s that made my car act funny and were difficult to diagnose. Way different setup, but my only point is that choppy reading really could be a bad O2.

Incidentally, idle sounded high to me. Then I went back and noticed your gauge read 1200. I don't trust stock gauges as far as I can throw them, but if you are idling as high as it sounds to me, then that's probably a vacuum leak or the idle adjustment screw needs to come back out. Don't see how else it's getting that much air. I know you said you smoke tested.

What's timing at?

I don't think that motor's hurt. It sounds alright aside from high idle and leak.

hmmm... that's fine for idle if that's with the spout out. That kind of RPM indicates no vac leak. If it's idling that low, it's probably surging because the computer is looking for it to idle at around 700. IIRC, the stock idle was just slightly less. I think you'll be ok in the 700-900 range. So, there goes my theories. Hope it's the O2s.

If you need a set of original low miles '88 injectors (19 lb), let me know.