ECU & Components:ECU Codes

There will be a diagnostic code inside the ECU (Electronic Control Unit) if and only if the 'Check Engine' light on the lower left portion of the dash is illuminated. To get the code that the ECU is reporting follow these steps:
  1. Locate the engine diagnostic test terminal located at the top of the drivers side kick panel (next to the fuse box). See illustration
  2. You will want to count the pulses that are produced from the MPI terminal. The easiest way I found was to buy a 12v electric buzzer (Radio Shack #273-055 $2.49) and attach its leads to the terminal connector, then just listen for the pulses. You can also use an analog voltmeter and count the pulses displayed.
  3. There are two types of pulses we are looking for "long" and "short". We are looking for a pattern where we have a series of long pulses (1 or more) and a series of short pulses (1 or more), then more long pulses. The codes always start with a long pulse and will finish with a short pulse. So start counting with the first long pulse after a short one. Count until you hear a long pulse after a short one again. NOTE: If you hear a constant "buzz", then your ECU is reporting that there is a problem with the ECU itself, if you hear only short pulses, there is no error code to report (i.e. normal state).
  4. Lets say we write down: S-S-L-L-S-S-S-L-L. We know that we start and stop with a long pulse after a short one. So that happens at two places in our example. At the 2nd/3rd pulse set, and at the 7th/8th pulse set. The pulses between those two are what we want: L-L-S-S-S.
  5. Now that you have the code, we need a code number. For every long pulse count it as a 10, and every short pulse counts as 1. So in our example, it is 10-10-1-1-1. Add them up and that is your code number, in our example 23.
  6. Find your code number below and the resulting diagnostic item.

Self-Diagnosis Codes for 2.4L and 3.0L F.I. Trucks
Code #Diagnostic Item
11O2 sensor
12Volume air flow sensor
13Intake air temperature sensor
14Throttle position sensor
15Motor position sensor(Part of distributor)
21Engine coolant temperature sensor
22Crankshaft angle sensor
23No. 1 cylinder top dead center sensor
24Vehicle speed sensor(Reed switch)
25Barometric pressure sensor
36Ignition timing adjustment signal
42Fuel pump
43EGR (Ca. only)
55Idle air control valve position sensor(CA RWD from 1993 models)
59Heated oxygen sensor (rear)(CA RWD from 1993 models)
61Transaxle control unit cable for automatic transmission
62Warm-up control valve position sensor
short pulses onlyNormal state
Note: These codes are from the Haynes repair manual. Check with your local bookstore to confirm codes if your car's year is outside this range.

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