Ford Mustang EEC-IV decoding
1986-93

1986-93 Ford Mustangs use an EEC-IV ECA to control engine functions. Under ideal conditions, sensors any switches would last forever. Unfortunately, that is not the case. When something electronic fails, the dreaded "Check Engine" light comes on. Understanding why that light comes on is another story. Using simple code scanning tool will give you a code number. But what does that number mean? This decoder can help you out. Some problems assume that wiring is in good shape.

EEC-IV code	Test Type	Meaning	Problem
11	Both	System Pass	Nothing is wrong with the electronic system. If there is a problem, chances are it's mechanical.
12	KOER	System cannot raise engine speed	Check the IAC valve. It may be stuck or sticking.
13	KOER	Idle RPM out of specification	Idle is either too low or too high.
14	KOER	Intermittent loss of PIP signal	PIP or ignition module may be malfunctioning.
15	KOER	Failure in ECA assembly	There is a problem with the Keep Alive Memory. Basically, you need a new ECA.
16 or 17	KOER	RPM too low to run EGO test	The RPM is low causing the EGO sensor to sense rich. It can also be caused by an adjustable fuel pressure regulator set too high. If this is the case, reduce fuel pressure.
18	Both	Loss of TACH signal to ECA	Check the SPOUT connector.
19	KOEO	Failure in ECA assembly	There is a problem with the internal voltage regulator. Basically, you need a new ECA.
21	Both	ECT sensor out of range	Engine coolant wasn't brought up to normal operating temperature or ECT sensor is bad.
22	KOEO	MAP or BP sensor out of range	MAP or BP sensor is bad.
	KOER	MAP or BP sensor out of range	MAP/BP sensor is bad, or there is a vacuum leak.
23	Both	TP sensor out of range	TPS is either missadjusted or bad.
24	Both	ACT or VAT sensor out of range	ACT or VAT sensor is bad, or really dirty.
26	Both	MAF or VAF sensor out of range	MAF/VAF sensor is bad, really dirty, or not calibrated correctly (MAF).
27	Both	VSS signal voltage is too low	VSS is bad.
28	Both	VAT sensor out of range	VAT sensor is bad, or really dirty.
29	Both	VSS signal voltage is too low	VSS is bad.
31 or 32	Both	EVP sensor out of range	EGR is sticking or EVP is bad.
33	KOER	EVP sensor indicates EGR is not opening properly	EGR is bad, EVP is bad, and/or there is a vacuum leak.
34 or 35	Both	EVP sensor out of range	EGR is sticking or EVP is bad.
41	KOEO	Right EGO failure	EGO is bad, not connected, or missing.
	KOER	EGO indicates right bank lean	EGO is going bad, missing, or fuel pressure regulator is set too low.
42	KOER	EGO indicates right bank rich	EGO is going bad, fuel pressure regulator is set too high, or exhaust leak.
44	KOER	Right Thermactor Air System problem	Right Thermactor Air Control system switch is bad.
45	KOER	Thermactor air flow is always upstream	Related Thermactor switch is bad or vacuum leak in related vacuum lines.
46	KOER	Thermactor Air System unable to bypass air	Related Thermactor switch is bad or there is a vacuum leak in the line going to the valve closest from the smog pumps air output.
51	KOEO	ECT sensor out of range	ECT sensor is bad.
53	KOEO	TP sensor out of range	TP sensor is stuck/sticking or just plain bad. It would be a good idea to get a replacement.
54	KOEO	ACT sensor out of range	ACT sensor is really dirty or bad.
56	KOEO	MAF sensor out of range	MAF sensor is bad, really dirty, or not calibrated correctly. This code can also come up with a blow-through forced induction system (like Prochargers). Pro-M mass-airs will work with blow-though systems.
61	KOEO	ECT sensor is out of range	ECT sensor is bad or wire is grounded.
63	KOEO	TP sensor out of range	TP sensor is bad.
64	KOEO	ACT sensor out of range (signal voltage too low)	ACT sensor is bad or wiring is grounded.
66	Both	MAF sensor out of range (signal voltage too low)	MAF sensor is bad or not calibrated correctly.
67	KOEO	NDS circuit open	NDS is bad or wires are bad.
74	KOER	BOO switch not detected	Brake was not depressed when suppose to or BOO is bad.
75	KOER	BOO switch always closed circuit	BOO is stuck/sticking or is bad.
77	KOER	Operator error	Operator didn't execute WOT when told to during self test.
79	KOEO	A/C on during self test.	Turn A/C off for self test.
81	KOEO	TAD circuit failure	TAD is bad or circuit is bad.
82	KOEO	TAB circuit failure	TAB is bad or circuit is bad.
84	KOEO	EGR vacuum solenoid circuit failure	EGR vacuum solenoid is bad or circuit is bad.
85	KOEO	CANP solenoid circuit failure	CANP solenoid is bad or circuit is bad.
87	KOEO	Fuel pump relay circuit failure	Bad fuel pump relay or FP circuit is bad.
91	KOER	EGO indicates left bank lean	EGO is going bad, fuel pressure regulator is set too low, or bad left bank injector(s).
92	KOER	EGO indicates right bank lean	EGO is going bad, fuel pressure regulator is set too low, or bad right bank injector(s).
94	KOER	Thermactor Air System problem on right bank	Air passages for Thermactor Air System on the right side is clogged or right EGO is going bad.
95	KOEO	FPM signal indicates circuit failure	FPM senses fuel pump voltage is out of range.
96	KOEO	Fuel pump secondary circuit failure	FP secondary circuit is bad or fuel pump relay is bad.
98	KOER	ECA system problem; operating in FMEM	This is the dreaded "limp-mode". Extensive problems are what normally trip this code. Additional codes will be present with this code.

Glossary:
ACT	Air Charge Temperature sensor
BOO	Brake On/Off switch
BP	Barometric Pressure sensor
CANP	CANister Purge solenoid
ECA	Engine Control Assembly - also known as an ECM (Engine Control Management)
ECT	Engine Coolant Temperature sensor
EEC-IV	Electronic Engine Control system, version 4
EGO	Exhaust Gas Oxygen sensor - also known as an O2 sensor
EGR	Exhaust Gas Recirculation
EVP	EGR Valve Position sensor
FMEM	Failure Management and Effects Mode
FP	Fuel Pump relay
FPM	Fuel Pump Monitor
IAC	Idle Air Control
MAF	Mass Air Flow sensor
MAP	Manifold Air Pressure sensor
NDS	Neutral Drive Switch - also known as a Neutral Safety Switch
PIP	Profile Ignition Pick-up
SPOUT	SPark OUTput
TAB	Thermactor Air Bypass solenoid
TAD	Thermactor Air Diverter solenoid
TP	Throttle Position sensor - also known as TPS
VAF	Vain Air Flow sensor
VSS	Vehicle Speed Sensor

Some of these codes maybe be for other Ford vehicles using an EEC-IV ECA. Problem information is a suggestion only. Accuracy of the above information is not guaranteed.