www.jsamotors.com

It occured to me today that the ECU returns a simple series of digital pulses
for its error codes, so why couldnt you just hook up an LED to get the code...
Sure enough it works. This provides a nice alternative to having to spend
hundred of dollars on a computer diagnostic form any commercial diagnostic
stations or equipment.

Minimum parts needed:

1 LED any color (Reds are most common and cheapest)

Recommended parts:
1 LED
2 Aligator Clips (makes hooking up to diag port easier than haveing to hold
pins up to diag port)
    or
1 12pin female connector (not sure on the type of this, I'm sure they sell
them somewhere though)

Note: This will only work on 90 - 94 model Talons, Eclipses and Lasers.

The method described is using the aligator clips, since this is what I had at
my disposal.
Wire the aligator clips to the two leads on the LED (for standard color coding
sake, hook a Red Aligator Clip up to the longer lead (ie the Anode or Positive
lead) and a Black Aligator clip to the shorter (Cathode or Negative) lead.

The diagnostic port, in case you didnt know, is under the drivers side dash
board, directly to the right of the fuse box. Now hook the Longer lead(Anode,
Red if correct color clips used) to pin 1 of your diagnostic port. Than hook
the other lead (Cathode, Black, shorter one) to pin 12. The pin configuration
for the diagnostic port is as follows:

1   2
3   4
5   6
7   8
9   10
11  12

not all leads are there, but pin 1 is in the top left corner of the diag port
connector, and pin 12 is on the bottom right corner. Once you hook up the led
to the connector, turn your ignition key to the on position (do not start your
car), if the LED blinks than it worked, if not you either have it hooked up
backwords, on the wrong ports, have the wires wrong on the LED, or have a
blown LED. Be very careful not to hook up the LED backwords, since LEDS have a
low reveresed-bias breakdown voltage (ie they blow easily if hooked up wrong).
Once hooked up you get one quick flash, followed by the error code. Basically
these are two digit number representations, where long flashes are in the 10s
place holder, and short flashes are numbers in the 1's place holder. Example,
if you get 2 long flashes, a pause, then two short flashes the error code is
interpreted as error 22 (Crankshaft position sensor). The error codes are as
follows:

Legend:
L = Long Flash
S = Short Flash
P = Pause

Numeric Error Code  Flash of LED            Description of Error
11                  L-P-S                   Heated Oxygen Sensor
12                  L-P-S-S                 Volume Air Flow Sensor
13                  L-P-S-S-S               Intake Air temperature Sensor
14                  L-P-S-S-S-S             Throttle Position Sensor
15                  L-P-S-S-S-S-S           Idle Speed Control Motor Position Sensor (1.8L)
21                  L-L-P-S                 Engine Coolant Temperature Sensor
22                  L-L-P-S-S               Crankshaft Position Sensor
23                  L-L-P-S-S-S             Camshaft Position Sensor
24                  L-L-P-S-S-S-S           Vehicle Speed Sensor (reed switch)
25                  L-L-P-S-S-S-S-S         Barometric pressure Sensor
31                  L-L-L-P-S               Knock Sensor (only on 2.0L turbo)
36                  L-L-L-P-S-S-S-S-S-S     Ignition Timing adjustment signal (1.8L)
41                  L-L-L-L-P-S             Injector
42                  L-L-L-L-P-S-S           Fuel Pump
43                  L-L-L-L-P-S-S-S         EGR (California Models only)
44                  L-L-L-L-P-S-S-S-S       Ignition Coil, Ignition Power Transistor (2.0L)

Exceptions: There are only two error codes not listed above that are
exceptions. If the LED stays constantly on (ie does not blink at all) then the
ECU is dead. if the LED constantly does short blinks, than there are no error
codes reported. One to to remember is that all error codes are retained in
memory except for Dead ECU, Normal State, and error 36 Ignition timeing
adjustment signal.

Thats pretty much it. you can do what you like with this, you can solder the
led and clips onto a project board and mount them in a project box to have a
nice looking test unit, or you can rum wires from the diag pins onto your dash
to have a dash mounted error readout (this might be annoying due to the fact
that the cycle continues constantly and youll have to put up with a flashing
led all of the time).

[I would caution readers that they include at least a 470 ohm resistor in-line
 with the LED.  There are LEDs with resistors inline that might not need
 an external resistor and I think that is what this poster used.  Several
 people have blown-out the solenoid output of their ECU by not including
 a resistor for the knock indicator hack.  Perhaps the ECU has some current
 limiting on the diagnostic "ping-pong" output, but I'm not sure about that.

Information Source: www.dsm.org