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Before we start, there is one important thing that you need to keep in mind. You're asking a computer what is wrong with your car. The computer will look at data from its sensors, and tell you what sensors are giving it 'bad' data. What it doesn't tell you is why the sensors are giving it bad data -- the sensor could be bad, or a connector could be loose, or corroded, or a wire could be broken or shorted, or the sensor could be accurately reporting a problem that is unrelated to the actual sensor. So before blindly purchasing a new sensor, stop to think if what the computer is telling you makes sense.

Having said that:

These instructions assume:
  • You have a Malfunction Indicator Lamp (MIL) and that it is operational
  • Your car has the 12v V6 engine. These codes may not accurately translate to different OBD-I versions, such as those used with the 5 cylinder engine.

If you do not have an operational MIL (commonly known as a 'check engine light' ... it's a picture of an engine with a lightning bolt through it, and should be one of only a couple of amber warning lights), you need to use the test light method.

Now find the connectors you need. They are located in the fusebox, which on most models is under the hood behind the firewall, on the driver's side. Open the fusebox and look for a pair of two-pin connectors in corner:
Data Link Connector

If the engine runs, you should try to extract and view the trouble codes with the engine running. If the engine will not run, crank the engine for 5 or 6 seconds (never crank the engine for more than 10 seconds!) and leave the key in the 'on' position. Now you need to make a connection between one pin in each connector. In the picture below, carefully note the position, shape, and orientation of the two data link connectors: one is white, and the other is black. They both have keyed edges, but they are keyed differently. Their location in the fuse box varies between models, as does the orientation of the fuse box itself, so look closely at the actual connectors rather than where they are in the box. Connecting the wrong pair of pins is unlikely to cause any damage, but you might inadvertantly clear the codes (so you'll have to wait for them to appear again). You should connect the two pins for 3 seconds using a short length of wire (I usually use a bent paper clip):

Check the codes
Thanks to Kent McLean for correcting this photo

Once you have removed the wire, immediately go look at the instrument cluster, and watch the MIL. It will start blinking a sequence -- each trouble code will be comprised of 4 groups of flashes with a maximum of 4 flashes per group. Between each group of four (one code), there will be a delay of about 2.5 seconds before the next code starts to come out:
Code flashes

If no trouble codes were present, the code "4444" will display. If the engine is not running when you view the codes, the first code you will see will be 2111. This code (engine speed sensor) can safely be ignored if the engine is not running when the codes are pulled. When all of the codes are displayed, the code 0000 (end of output) is displayed -- 0000 is represented by four 2.5 second flashes at 2.5 second intervals. Once this has displayed, you can decode the 4-digit code to see what the problem is by consulting the table below.

On some 1992-1993 models (those with the MMS200 ECU), if you have multiple DTCs stored you need to momentarily connect the pins (the ones you just shorted) to advance from the currently displayed code before the next code will be displayed. (Thanks to John Dolen for pointing this out!)

When you have looked at the codes, fixed the problems, and are ready to test your work, you can clear all of the DTCs by shorting the other two pins on the two connectors for three seconds:
Clear the codes

No DTC recognized.

Display ends.

Vehicle Speed Sensor.

Engine Speed Sensor.

Crankshaft Position Sensor.

Camshaft Position Sensor.

Closed Throttle Position Switch.

Throttle Position Sensor.

CO Fuel Trim Potentiometer.

Engine Coolant Temperature Sensor [G62].

Knock Sensor 1.

Heated Oxygen Sensor 1 [G39].

Mass Air Flow Sensor Reference Voltage.

ECM Supply Voltage.

Knock Sensor Control 1.

Knock Sensor Control 2.

Oxygen Sensor Control 1.

Knock Sensor 2.

Mass Air Flow Sensor.

Oxygen Sensor Control 2.

Heated Oxygen Sensor 2 [G108].

EGR System.

Ignition Output 1, 2, or 3.

EVAP Canister Purge Regulator Valve [N80].

Injector, Cylinder 1 [N30].

Injector, Cylinder 2 [N31].

Injector, Cylinder 3 [N32].

Injector, Cylinder 4 [N33].

Injector, Cylinder 5 [N83].

Injector, Cylinder 6 [N84].

Idle Air Control Valve [N71].

EGR Vacuum Regulator Solenoid Valve.

Control Module Malfunction.
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