The fox Mustang’s EEC IV computer has two connectors for self-testing and located on the driver-side firewall. The large connector contains the self test output (STO) and ground (SIG RTN). The small pigtail is the self test input (STI). You can easily grab your trouble codes, and this will help you diagnose your MIL or “Check engine ” light on that fox body Mustang of yours! Note: Make sure your engine is warmed up before doing these tests.

Counting Digits to Get Your Trouble Codes

If you don’t have a code-scanner, you can count the signals and find the codes that way. It may take patience to do so but doable. To get a code digit: There’s a 2 second pause between digits and 4 second pause between codes. So for code example “21”, you will count two flashes, a 2 second pause, then a single flash, if there’s another code – it will pause 4 seconds.

Using a Test Light (86-88 Mustangs)

The 86 to 88 Mustangs will need a jumper wire from the STI (test input) with the #2 pin on the connector (I’d use spades) and you can hook your light to #4 on the connector and (+) to the positive battery terminal.

Using test light trouble codes fox Mustang EECIV

Using The Check-Engine Light (89-93 Mustangs)

The 89-93 Mustangs (and the 94-95’s as well as some ’88 MAF equipped Mustangs) will allow you to use a jump wire and count the check-engine light in the dash. The jump wire (I’d use spades on each end) would connect the STI (test input) with #2 pin on the connector, see below:

EEC trouble code reading - jump wire

Test Conditions and EEC-IV Codes Below:

(O) =Key On Engine Off  (KOEO)
(R) =Key On Engine Running (KOER)
(M) =Memory code
(C) = Continuous memory

2 Digit Codes

11O,R,CSystem OK, testing complete
12RIdle speed control out of specified range
13O,R,CNormal idle not within specified range
14O,CIgnition profile pickup erratic
15OROM test failure
15CPower interrupt to computer memory
16RErratic idle, oxygen sensor out of range or throttle not closing
17RCurb idle out of specified range
18RSPOUT circuit open
19ONo power to processor
19RErratic idle speed or signal
21O,RCoolant temperature out of specified range
21O,R,CCoolant temperature sensor out of specified range
22O,R,CMAP sensor out of specified range
23O,R,CThrottle position signal out of specified range
24O,RAir charge temperature low
25O,RMass Air Flow sensor or circuit
27CVehicle Speed Sensor or circuit
280,RVane air temperature sensor or circuit
29CNo continuity In Vehicle Speed Sensor circuit
31O,R,CCanister or EGR valve control system
32O,R,CCanister or EGR valve control system
33R,CCanister or EGR valve not operating properly
34O,R,CEVP voltage above closed unit
35O,R,CEGR pressure feedback, regulator circuit
38CIdle control circuit
39CAutomatic overdrive circuit
41COxygen sensor signal
41RLean fuel mixture
42R,CFuel mixture rich
43CLean fuel mixture at wide open throttle –
43REngine too warm for test
44RAir management system inoperative
45RThermactor air diverler circuit
46RThermactor air bypass circuit
47RLow flow of unmetered air at idle
48RHigh flow of unmetered air at idle
49CSPOUT signal defaulted to 1 0-degrees
51O,CCoolant temperature sensor out of specified range
52O,RPower steering pressure switch out of specified range
53O,CThrottle Position Sensor input out of specified range
54O,CVane air flow sensor or air charge temperature sensor
55RCharging system under specified voltage (1984 through 1988)
55ROpen ignition key power circuit (1984 through 1988)
56O,R,CMass Air Flow sensor or circuit
57CTransmission neutral pressure switch circuit
58OCFI – idle control circuit; EFI – vane air flow circuit
58RIdle speed control motor or circuit
58CVane air temperature sensor or circuit
59O,CTransmission throttle pressure switch circuit
61O,CCoolant temperature switch out of specified range
620Transmission circuit fault
63O,CThrottle Position Sensor or circuit
64O,CAir Charge Temperature (ACT) sensor
65CFuel control system not switching to closed loop
66O,CNo Mass Air Flow sensor signal
67O,R,CNeutral drive switch or circuit
67CAir conditioner clutch switch circuit
69O,CVehicle Speed Sensor or circuit
72CSystem power circuit, electrical interference
72RNo Manifold Absolute Pressure or Mass Air Flow sensor signal fluctuation
73O,RThrottle Position Sensor or circuit 21
74RBrake on/off ground circuit fault 21
75RBrake on/off power circuit fault 22
76RNo vane airflow change
77RThrottle “goose” test not performed
78CPower circuit 1
79OAir conditioner clutch circuit
81OThermactor air circuit
82OThermactor air circuit, integrated controller circuit
83OEGR control circuit (four-cylinder models oeM
83OCooling fan circuit (V6 models only)
83O,CLow speed fuel pump relay (1984 through 1988 models)
83O,CEGR solenoid or circuit (1989 and later models)
84O,REGR control circuit
85O,RCanister purge circuit or transmission shift control circuit
85CExcessive fuel pressure or flow
85CCanister purge circuit
86CLow fuel pressure or flow
87O,R,CFuel pump circuit D2
88OIntegrated controller
89OLock-up solenoid
91R,COxygen sensor problem, fuel pressure out of specified range or injectors out of balance
92RFuel mixture rich, fuel pressure high
93OThrottle Position Sensor or circuit D
94RSecondary air system inoperative
95O,CFuel pump circuit problem
95RThermactor air diverter circuit
96O,CFuel pump circuit
96RThermactor air bypass circuit
98RRepeat test sequence
99RRepeat test sequence
99RSystem has not learned to control idle speed

3 Digit Codes (some 91+)

111O,R,CSystem Pass
112O,R,CAir Charge Temp (Ad) Sensor circuit below minimum voLtagE/ 254 F
113O,CAir Charge Temp (ACT) Sensor circuit above maximum voltagE/ -40 F
114O,RAir Charge Temp (ACT) out of self test range
116O,RCoolant Temp (ECT) out of self test range
117O,RCoolant Temp (EdT) Sensor circuit below minimum voltage/ 254 F
118O,CCoolant Temp (EdT) Sensor circuit above maximum voitage/ -40 F
121O,CClosed Throttle Voitage higher or lower than expected
122O,R,CThrottle Position (TP) Sensor circuit below minimum voltage
123O,CThrottle Position (TP) Sensor circuit above maximum voltage
124O,CThrottle Position (TP) Sensor voltage higher than expected
125CThrottle Position fTP) Sensor voltage lower than expected
126CBP Sensor higher or lower than expected
129O,R,CInsufficient Mass Air Flow (MAE) change during Dynamic Response Test
144CNo Oxygen Sensor (HEGO) Switches detected
157CMass Air Flow (MAE) Sensor circuit below minimum voltage
158O,CMass Air Flow (MAE) Sensor circuit above maximum voltage
159O,RMass Air Flow (MAE) Sensor out of self test range
167RInsufficient Throttle Position change during Dynamic Response Test
171CFuel system at adaptive limits, Oxygen Sensor (HEGO) unable to switch
172R,CLack of Oxygen Sensor (HEGO) Switches, indicates lean
173R,CLack of Oxygen Sensor (HEGO) Switches, indicates rich
179CFuel system at lean adaptive limit at part throttle, system rich
181CFuel system at rich adaptive limit at part throttle, system lean
182CFuel system at lean adaptive limit at idle, system rich
183CFuel system at flch adaptive limit at idle, system lean
184CMass Air Flow (MAE) higher than expectec
185CMass Air Flow (MAE) lower than expected
186CInjector pulse width higher than expected
187CInjector pulse width lower than expected
211CProfile Ignition Pickup (PIP) circuit fault
212CIgnition module circuit failure – SPOUT circuit grounded
213RSPOUT circuit
214CCylinder Identification (CID) circuit failure
215CEEC Processor detected Coil 1 primary circuit failure
216CEEC Processor detected Coil 2 primary circuit failure
218CLoss of ignition Diagnostic Monitor (1DM) signal-left side
222CLoss of Ignition Diagnostic Monitor (1DM) signal-right side
223CLoss of Dual Plug Inhibit (DPI) control
224CErratic Ignition Diagnostic Monitor (1DM) input to processor
225RKnock riot sensed during Dynamic Response Test
327O,R,CEVP circuit below minimum voltage
328O,R,CEGR “closed valve” voltage lower than expected
332R,CInsufflcjent EGR flow detected
334O,R,CEGR closed valve voltage higher than expected
337O,R,CEVP circuit above maximum voltage
341OOctane Adjust Service Pin in use
411RCannot control RPM during KOER low rpm check
412RCannot control rpm during KOER high rpm check
452CIsufflcient input from Vehicle Speed Sensor (VSS)
511OEEC Processor Read Only Memory (ROM) test failure
512CEEC Processor Keep 4Jive Memory (KAM) test failure
519OPower Steering Pressure Switch (PSPS) circuit open
521RPower Steering Pressure Switch (PSPS) circuit did not change states
522OVehicle not in PARK or NEUTRAL during KOEO
528CClutch Switch Circuit failure
536R,CBrake On/Off (BOO) circuit failure I not actuated during KOER
538RIsuffIcient RPM change during KOER Dynamic Response Test
539OAC On/Defrost ON during KOEO
542O,CFuel Pump secondary circuit failure
543O,CFuel Pump secondary circuit failure
556O,CFuel Pump Relay primary circuit failure
558OEGR Vacuum Regulator (EVR) circuit failure
5640ElectrOD FAN( EDF) circuit failure
565OCanister Purge (CANP) circuit failure
566O3-4 Shift Solenoid circuit failure
629OConverter Clutch Control circuit failure
998O,RHard Fault present