ECU Code Checking

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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:

Locate the engine diagnostic test terminal located at the top of the drivers side kick panel (next to the fuse box). See illustration
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.
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).
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.
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.
Find your code number below and the resulting diagnostic item.

Self-Diagnosis Codes for 1.8L and 2.0L Engines
Code #	Diagnostic Item
Constant	ECU
11	O₂ sensor
12	Volume air flow sensor
13	Intake air temperature sensor
14	Throttle position sensor
15 (1.8L only)	Idle speed control motor position sensor
21	Engine coolant temperature sensor
22	Crankshaft position sensor
23	Camshaft position sensor
24	Vehicle speed sensor
25	Barometric pressure sensor
31 (2.0L Only)	Knock sensor (turbo only)
36 (1.8L Only)	Ignition timing adjustment signal
41	Injector
42	Fuel pump
43	EGR (Ca. only)
44 (2.0L Only)	Ignition coil, ignition power transisitor unit
short pulses only	Normal state

Note: These codes are from the 1990-1994 repair manual. Check with your local bookstore to confirm codes if your car's year is outside this range.

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