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OBD1 Code Guide & Jumper Procedure For Your Fox Body Mustang

Created by Jay Walling / 8 min read
Date Created: 5/12/2020
Last Updated: 2/7/2023

Check out LMR's run-through of all the OBD1 codes for your Fox Body Mustang! This will help you diagnose your vehicle quickly and efficiently.

Viewing this install and using the information shared is subject to the terms set forth here - View the LMR Install Instructions Disclaimer.

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Fox Body Mustang OBD1 Codes - Fox Body Mustang OBD1 Codes

Often, when driving your Fox Body Mustang, so many different issues can come about at any time that can affect your vehicle's health. We at LMR have created this Fox Body Mustang OBD1 code guide and jumper procedure to better help you find and solve your specific problem without buying an expensive diagnostic code reader. Check out our guide below!

Fox Body Mustang OBD1 Guide

Service Code Service Code Definition Service Code Service Code Definition
11 ORC System PASS 51 OC ECT indicated -40°C (-40°F)/circuit open
12 R Cannot control rpm during Self-Test high rpm check 53 OC TP circuit above maximum voltage
13 R Cannot control rpm during Self-Test low rpm check 54 OC ACT indicated -40°C (-40°F)/circuit open
14 C PIP circuit failure 56 OC MAF circuit above maximum voltage (MA only)
15 O EEC processor Read Only Memory (ROM) test failed 61 OC ECT indicated 123°C (254°F)/circuit grounded
15 C EEC processor Keep Alive Memory (KAM) test failed 63 OC TP circuit below minimum voltage
16 R Rpm too low to perform EGO test 64 OC ACT indicated 123°C (254°F)/circuit grounded
18 R SPOUT circuit open 66 C MAF circuit below minimum voltage (MA only)
18 C IDM circuit failure/SPOUT circuit grounded 67 O Neutral Drive Switch (NDS) circuit open - A/C ON during Self-Test
19 O Failure in EEC processor internal voltage 74 R Brake On/Off (BOO) circuit open/not actuated during Self-Test
21 OR ECT out of Self-Test Range 75 R Brake On/Off (BOO) circuit closed/EEC processor input open
22 ORC MAP/BP out of Self-Test range 77 R Brief WOT not sensed during Self-Test/Operator error
23 OR TP out of Self-Test range 79 O A/C on/Defrost on during Self-Test
24 OR ACT out of Self-Test range 81 O Air Management 2 (AM2) circuit failure
26 OR MAF out of Self-Test range (MA only) 82 O Air Management 1 (AM1) circuit failure
29 C Insufficient input from the Vehicle Speed Sensor (VSS) 84 O EGR Vacuum Regulator (EVR) circuit failure
31 ORC EVP circuit below minimum voltage 85 O Canister Purge (CANP) circuit failure
32 ORC EVP voltage below closed limit 87 OC Fuel pump primary circuit failure
33 RC EGR valve opening not detected 91 R HEGO sensor circuit indicates system lean (left HEGO)
34 ORC EVP voltage above closed limit 91 C No HEGO switching detected (left HEGO)
35 ORC EVP circuit above maximum voltage 92 R HEGO sensor circuit indicates system rich (left HEGO)
41 R HEGO sensor circuit indicates system lean (right HEGO) 94 R Thermactor air system inoperative (left side)
41 C No HEGO switch detected (right HEGO) 95 OC Fuel pump secondary circuit failure
42 R HEGO sensor circuit indicates system rich (right HEGO) 96 OC Fuel pump secondary circuit failure
44 R Thermactor air system inoperative (right side) 98 R Hard fault is present - FMEM mode
45 R Thermactor air upstream during Self-Test NO CODES Unable to initiate Self-Test or unable to output Self-Test codes
46 R Thermactor air not bypassed during Self-Test CODES NOT LISTED Service codes displayed are not applicable to the vehicle being tested
KEY: 0 = Key On Engine Off (KOEO) | R = Engine Running (ER) | C = Continuous Memory

How To Pull Diagnostic Codes on 1986-93 Mustangs

Often on your Mustang, codes can be present even if the check engine light hasn't been activated, so LMR has put together a list of steps a cheap and easy way to dump the computer codes without using expensive equipment. Also, please be aware that trouble codes are 2 or 3 digits. No cars use both digits, so be aware of that during your diagnosis.

What You Will Need
  • Jumper wire or paper clip
  • Check engine light, voltmeter, or test light

Dumping Codes With Engine Turned OFF

  1. Turn off the A/C
  2. When dumping the codes, put the transmission in neutral, and if it's a manual transmission, be sure to press the clutch to the floor. If you do not do this, you will generate a code 67 (Neutral Drive Switch (NDS) circuit open - A/C ON during Self-Test) and won't be able to dump the engine running codes.
  3. Please note on the 1986-88 Mustangs, the voltmeter or test lamp method will need to be used due to the non-functional check engine light.

  4. Fox Body Mustang OBD1 Codes - Fox Body Mustang OBD1 Codes Fox Body Mustang OBD1 Codes - Fox Body Mustang OBD1 Codes

  5. Don't jump the Single Dark Brown Connector With Black/Orange Wire. This is the 12-volt power to the underhood light and cannot be used as a jumper to the computer test connect. If done, the computer will be damaged.
  6. If you have performed the steps correctly, you should get a code 11 (System PASS). If code 11 doesn't pop up, you might have some wiring problems.
  7. Also, be aware that the same wire providing the ground to dump the codes provides the signal ground for the TPS, ACT, EGR, and Map/Baro sensors. If for some reason it fails, many different performance issues will come about.
  8. There are some codes that have different answers if the engine is running vs. not running, so it helps to know if it was running or not to perform the correct diagnosis.

Dumping Codes With Engine Turned ON

  1. Very similar to running with it off, you will start the engine with the test jumper in the correct position.
  2. Make sure that the A/C is off, and clutch (if available) is pressed to the floor, and the transmission is in neutral.
  3. You should be presented with a code 11 (System PASS).
  4. Then a four will be presented, and the engine will speed back up to perform the EGR test.
  5. Once the engine speed comes down to idle, it will dump the remaining engine running codes.

That's it! Dumping your codes can be quick and easy if provided with the correct steps. As always, for all things engine-related for your Fox Body Mustang, check out all of our products at Late Model Restoration.


Thumbnail image of the author of this article, Jay Walling.

About the Author

Jay has written content for Late Model Restoration for over 10 years, producing over 120 articles. Jay has an extensive 25-plus-year background in automotive and is a certified Ford Technician. Read more...