1989 Gt O2 Sensor

The fact that your car ran fine without O2 sensors seems strange. You should check for computer trouble codes.

Wen you disconnect the O2 sensors, the computer goes into Limp Mode and does not depend on sensor input. Limp Mode is to just that; limp to the nearest shop to fix the problem. It uses safe settings to prevent damage to the engine. Limp Mode gives reduced performance and economy.



Dump the codes: Codes may be present even if the Check Engine Light (CEL) isn't on.

Dumping the computer diagnostic codes on 86-95 Mustangs

Revised 26-July-2011. Added need to make sure the clutch is pressed when dumping codes.

Codes may be present even if the check engine light hasn’t come on, so be sure to check for them.

Here's the way to dump the computer codes with only a jumper wire or paper clip and the check engine light, or test light or voltmeter. I’ve used it for years, and it works great. You watch the flashing test lamp or Check Engine Light and count the flashes.

Post the codes you get and I will post 86-93 model 5.0 Mustang specific code definitions and fixes. I do not have a complete listing for 94-95 model 5.0 Mustangs at this time.

Be sure to turn off the A/C, and put the transmission in neutral when dumping the codes. On a manual transmission car, be sure to press the clutch to the floor.
Fail to do this and you will generate a code 67 and not be able to dump the Engine Running codes.





If your car is an 86-88 stang, you'll have to use the test lamp or voltmeter method. There is no functional check engine light on the 86-88's except possibly the Cali Mass Air cars.



The STI has a gray connector shell and a white/red wire. It comes from the same bundle of wires as the self test connector.

89 through 95 cars have a working Check Engine light. Watch it instead of using a test lamp.



The STI has a gray connector shell and a white/red wire. It comes from the same bundle of wires as the self test connector.


WARNING!!! There is a single dark brown connector with a black/orange wire. It is the 12 volt power to the under the hood light. Do not jumper it to the computer test connector. If you do, you will damage the computer.

What to expect:
You should get a code 11 (two single flashes in succession). This says that the computer's internal workings are OK, and that the wiring to put the computer into diagnostic mode is good. No code 11 and you have some wiring problems. This is crucial: the same wire that provides the ground to dump the codes provides signal ground for the TPS, EGR, ACT and Map/Baro sensors. If it fails, you will have poor performance, economy and driveablity problems

Some codes have different answers if the engine is running from the answers that it has when the engine isn't running. It helps a lot to know if you had the engine running when you ran the test.

Dumping the Engine Running codes: The procedure is the same, you start the engine with the test jumper in place. Be sure the A/C is off, and clutch (if present) is pressed to the floor, and the transmission is in neutral. You'll get an 11, then a 4 and the engine will speed up to do the EGR test. After the engine speed decreases back to idle, it will dump the engine running codes.

Trouble codes are either 2 digit or 3 digit, there are no cars that use both 2 digit codes and 3 digit codes.

Your 86-88 5.0 won't have a working Check Engine Light, so you'll need a test light.
See AutoZone Part Number: 25886 , $10




Alternate methods:
For those who are intimidated by all the wires & connections, see Actron® for what a typical hand scanner looks like. Normal retail price is about $30 or so at AutoZone or Wal-Mart.

Or for a nicer scanner see Equus Digital Ford Code Reader (3145) Equus - Digital Ford Code Reader 3145.
It has a 3 digit LCD display so that you don’t have to count flashes or beeps.. Cost is $22-$36.

The coil wires should read 12v when putting one lead on each wire with key on.

Amador124 said:

Should both wires read positive voltage when connected to the battery? When I connect the ground to the battery and test both wires going to the coil, they both have positive voltage. Is one supposed to have negative voltage?

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You are chasing the wrong rabbit, I advise you to go back and dump the codes.
Looking for an O2 sensor problem by measuring the voltage at the coil will not get you anywhere useful.

The coil wires question: the red/green wire will read 12 volts. The other wire will read a pulsing voltage when the engine is cranking..

jrichker said:

You are chasing the wrong rabbit, I advise you to go back and dump the codes.
Looking for an O2 sensor problem by measuring the voltage at the coil will not get you anywhere useful.

The coil wires question: the red/green wire will read 12 volts. The other wire will read a pulsing voltage when the engine is cranking..

Click to expand...

i am having the same problems with mine i am going to get the scanner now from autozone

hsean said:

i am having the same problems with mine i am going to get the scanner now from autozone

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What? you led your horse to water and can't get it to drink?