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Since this comes up so often, and the procedure can't be found, or isn't looked for, I decided to write this. The info comes from a Haynes manual, and I'm including the proper testing procedure, what the codes mean, and how to build a code tester. This is all rudimentary stuff, and easy to do. Of course it's easier to take the car to a mechanic and have this done, but that requires money and more time than doing the test alone.

Background:
The EEC-IV ystem has a self-diagnosis capability that stores trouble codes in the ECU (computer) that can be retrieved and help identify problem areas in the system. Usually, but not always, an error code is stored in the ECU when the "Check engine light"(CEL) comes on. In the engine compartment, find the "Self Test" connector. The connector has two parts: a large one with six output terminals and a single input terminal. The connector is located on the right (passenger) side of the firewall, near the strut tower.

DIY code checker:
Obviously the code checkers are available for purchase. However, I bought one years ago, and it didn't actually work as the test pins were in the wrong location. It was a basic Actron tester ($30 USDat the time). Shortly after I discovered that I didn't need it. There are two steps to constructing a DIY test light.

1.You will need to make a "jumper" wire. Both ends must be spade type connectors, that is, flat on the ends to fit in the test connectors. I use a piece of plain copper wire from house wiring that has been flattened on both ends. It's crude, but effective. The better method is to take a short length of wire (no more than 6 inches) and crimp spade connectors on both ends. That's all there is to making the test jumper.
2.A test light is not needed, but some prefer it. That can either be a 12 volt light bulb or LED (with accompanying resistor) with one end connected to the test output connector, and the other to the POSITIVE battery post. Another way is to use an analog multimeter set to read 12 volts. The CEL (or upshift indicator in one case) will also flash when checking for error codes. I have personally used a 12 volt light that was in a twist in socket meant for dash lights. I soldered on wires to either post of the socket, and put spade connectors on. Very simple, and it works fine. The cost of making this is pennies. If you've an analog multimeter sitting around, you're good to go.
Next you get to plug the jumper into the single input port from the top right connector on the large harness. The test light goes from the second connector from the left to the positive battery post. A picture is included to prevent confusion. Just follow the diagram and you'll have everything hooked up properly.



Now that the test "jumper" and "light" are ready, or if you're just using the CEL (or upshift light if your car is weird) we'll move onto the meat and potatoes of this how-to. There are three modes to testing: Key On Engine Off (KOEO), Continuous Memory ( C ), and Engine Running ( R ).

KOEO Test:
Turn on the ignition and watch the CEL/Voltmeter/Light, The codes are displayed as flashes or sweeps of the needle. Example: Two flashes/sweeps followed by three flashes/sweeps indicates code 23. There is a 4 second delay between the codes. Write them down so they're not forgotten. Codes appear in ascending numerical order (smallest to largest), with Code 11 being "All Clear." Codes repeat in order twice.

C Test:
After the KOEO codes are posted there is a short pause and the C codes are read from memory. Again, code 11 is "all clear."

R Test:
This time you start the engine, and it will rev fairly high. Don't panic, that is simply the ECU testing that the ignition system can advance the timing properly. If you check the timing it should be 20 degrees advanced beyond base timing (8 degrees is base timing). In other words it should read 28 degrees advanced. Shut the engine off, restart it, and let it run for 2 minutes, then turn it off for ten seconds before restarting again. There should be some quick flashes/sweeps (this is the engine code the ECU recognizes). Two sweeps means you have a 4 cylinder (but you already knew that). After another pause there will be a single flash/sweep, which is the signal for you to tap the gas pedal so the throttle system can be checked. This is known as the "throttle goose test." After this will be a pause, followed by codes. They still repeat in order twice.

It's a good idea to have the engine at operating temp (let the cooling fan kick on and once it shuts off you're at operating temp). If the engine is below operating temp, the ECU will output a code that the water and air temperature sensors are out of specification. So don't panic if those codes come up.

Codes:
11- All Clear
12- Idle speed control out of specified range
13- Normal idle not within specified range
14- Ignition Profile Pickup erratic (PIP module..the orange piece inside the distributor)
15- ROM test failure
16- Erratic idle, O2 sensor out of range, or throttle not closing
17- Curb idle out of specified range
18- SPOUT circuit open
19- O TEST- No power to processor
19- R TEST- Erratic idle speed or signal
21- Coolant temp or sensor out of specified range
22- MAP sensor out of specified range
23- Throttle position signal out of specified range
24- Air charge temperature low
26- MAF/VAF sensor or circuit
27- Vehicle speed sensor or circuit
28- Vane air temp sensor or circuit
29- No continuity in speed sensor circuit
31- Canister or EGR valve control system
32- Canister or EGR valve control system
33- Canister of EGR valve not operating properly
34- Canister or EGR valve control circuit
35- EGR pressure feedback, regulator circuit
38- Idle control circuit
39- Automatic overdrive circuit
41- C TEST - O2 sensor signal
41- R TEST - Lean fuel mixture
42- Fuel mixture rich
43- C TEST - Lean fuel mixture at WOT
43- R TEST - Engine too warm for test
44- Air management system inoperative
45- Thermactor air diverter circuit
46- Thermactor air bypass system
47- Low flow of unmetered air at idle
48- High flow of unmetered air at idle
49- SPOUT signal defaulted to -10 degrees
51- Coolant temp sensor out of specified range
52- Power steering pressure switch out of specified range
53- Throttle position sensor out of specified range
54- VAF or air temp sensor
55- Charging system under specified voltage
55- Open ignition key power circuit
56- MAF sensor or circuit
57- Transaxle neutral pressure switch circuit
58- O TEST - CFI- Idle control circuit or EFI-VAF circuit
58- R TEST - Idle speed control motor (ICM) or circuit
58- C TEST - Air temp sensor or circuit
59- Transaxle throttle pressure switch circuit
61- Coolant temp switch out of specified range
62- Transaxle circuit failure
63- Throttle position sensor or circuit
64- Air char temp (ACT) sensor
65- Fuel control system not switching to closed loop
66- No MAF sensor signal
67- Neutral drive switch or circuit
67- C TEST ONLY - A/C clutch switch circuit
69- Speed sensor or circuit
72- C TEST - System power circuit, electrical interference
72- R TEST - No MAP or MAF sensor signal fluctuation
73- Throttle position sensor (TPS) or circuit
74- Brake on/off ground circuit fault
75- Brake on/off power circuit fault
76- No vane airflow change
77- "Throttle Goose Test" not performed
78- Power circuit
79- A/C clutch circuit
81- Thermactor air circuit
82- Thermactor air circuit, integrated controller circuit
83- EGR control circuit
84- EGR control circuit
85- C TEST ONLY - Excessive fuel pressure or flow
85- O TEST ONLY - Canister purge circuit
85- O and R - canister purge circuit or transaxle shift control circuit
86- Low fuel pressure or flow
87- Fuel pump circuit
88- Integrated controller
89- Lock-up solenoid
91- O2 sensor, fuel pressure out of range, or injectors out of balance
92- Fuel mixture rich or fuel pressure high
93- TPS or circuit
94- Secondary air system inoperative
95- Fuel pump circuit problem
95- R ONLY - Thermactor air diverter circuit
96- Fuel pump circuit
96- R ONLY - Thermactir air bypass circuit
98- Repeat test sequence
99- System has not learned to control idle speed yet.

Clearing the memory: disconnect the battery for a few minutes and the memory will be cleared. The ECU will also have to re-lean how to control the idle speed. This is normal. Reconnect the battery, start the engine, and let it idle for about 5 minutes and the ECU will have learned to idle. If the car is driven, the ECU still learns to idle, although it takes much longer.
 
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