The diagnostic link consists of a 6 pin female socket as pictured to the right. Although designed to accept the GM Service Department's diagnosis tool, the Tech-1, most home mechanics won't have this kit and will have to improvise as a result. This is where the bent paper clip comes into play.

This link is our key to unlocking the error codes stored by the factory ECU which will can run in four different modes. The first mode- the one that makes us all happy- is when nothing is connected to the socket and the system is fine. This is normal mode. There is also a 'limp home' mode that may be used when some sensor information is unavailable. The two that we are most interested in for the sake of this article the diagnostic modes- the vehicle service mode which generates a computer readable dump of a set of parameters maintained by the ECU which the Tech-1 will talk to and the flashing error mode which is of most help to us.

To have the ECU run in one of these modes a resistor is to be placed between certain pins. Luckily for us, to activate the flashing error mode all we need to do is create a short (for those of you interested, a resistor of 10k is used for vehicle service mode and 3.9k for 'limp home' or 'backup' mode).

If you cross reference the diagram above with the photo, it is easy to identify each pin in a realistic scenario. I have not gone to the effort of jamming a paper clip into each terminal for research purposes so I can not confirm or deny what mysteries this may unlock. I definitely don't recommend giving it a go.

Flashing Error Mode

The first step in the process of activating the flashing error code mode is to locate the diagnostic link which can be found tucked into the passenger side trim, beneath the dash. It is not uncommon for the link to already be outside of the trim as you would be hard pressed to find a VN that hasn't been to the mechanic's a few times. Once you have found the socket- it is the only one of its type on that side of the vehicle- it is best to orientate it so that it is as per both the above photo and diagram to avoid confusing the pins.

Once you have found the pin and prepared the paperclip (straighten it out and then bend into a 'U' shape), find pins 'B' and 'A' and create a short by by placing one end of the world's greatest diagnostic tool into pin 'B' and pin 'A'. Its worth giving the clip a bit of a wiggle to ensure each end is home.

It really is that simple. You will read about people making their own, more elaborate contraptions to create the short between pins 'B' and 'A' and there is nothing wrong with doing so. I personally see it as a little unnecessary as the paper clip does the job, is readily available and doesn't cause any damage.

Now we have created the short and entered Flashing Error Mode we have to read our codes. The first time I did this, I expected the check engine light to immediately burst to life and give up all of the ECU's dirty little secrets. It didn't happen. You must turn the ignition so that the all pre- start up lights are illuminated but it is not necessary to actually start the car.

After a short period of time the check engine light will begin flashing and now is the time to count the flashes and write the findings down. Each fault code consists of two digits with each flash separated by 1.2 seconds. A pause of 3.2 seconds occurs between each digit group. For instance two rapid flashes followed by a 1.2 second pause and three more rapid flashes would give an error code of 23. If there was a 3.2 second pause followed by three rapid flashes, a 1.2 second pause and three more rapid flashes this would be our second digit group (second error code) being 33. As a result our error codes would be 23 and 33.

Once you have the code (and hopefully not codes!) you can reset or clear the error codes of the ECU by disconnecting the battery for a short period of time. To check that the ECU is clear of logged codes, once again use the Flashing Error Mode and it should return the code 12. The problem isn't magically fixed- it is just no errors are currently logged.

But What Does It All Mean?!

One you have your codes, you can find out the meaning by checking the list below. Unfortunately at this time we do not have how- to's for each fault and I strongly recommend getting a hold of the relevant Gregory's manual to get a bit of a leg up on how you can solve the problem. Hopefully in time we will have a few more technical write ups on how to fix the common causes of the faults.

Code Explanation 

12 - No spark pulses at CPU or else good system, no codes.

13 - Oxygen sensor output remained at .35-.55 volts for more than one minute after warm up. Possible open circuit.

14 - Coolant sensor indicated a temperature above 130C for 3 seconds after engine ran for 20 seconds. Probably a short.

15 - Coolant sensor indicated a temperature below -30C for 3 seconds when MAT > - 13 or engine running over 1 minute. Probably open circuit.

21 - Throttle pos sensor above 2.5 volts for 2 seconds when engine speed below 1600 rpm 22 Throttle pos sensor below .2 volt for 2 seconds while engine running.

23 - MAT sensor shows < -30 degrees C for 3 seconds after engine running 1 minute or coolant > 30C. Probably an open circuit.

24 - No speed sensor pulses when engine between 2000-4000 rpm, throttle closed, high vacuum, not in neutral and all for 5 seconds.

25 - MAT sensor showed above 145 degrees C for 2 seconds after engine ran for over 1 minute. Possible short circuit.

31 - Unknown Error Code.

32 - Unknown Error Code.

33 - MAP sensor voltage too high (> 4.00 v). Possible vacuum leak to sensor or faulty sensor.

34 - MAP sensor voltage too low (< 0.25 v) with ignition on or engine running >1200 rpm and throttle open >20% 35 Closed throttle idle speed is more than 75 rpm above or below correct value for more than 45 seconds.

41 - No Crankshaft reference pulses. Ignition voltage < 11 volts etc.

42 - Open or short on EST or BYPASS line to ignition module.

43 - Engine Knock Sensor open or shorted.

44 - O2 sensor showed < 0.250 volt for over 20 seconds while operating closed loop 45 O2 sensor showed > 0.550 volt for over 50 seconds while in closed loop with engine running over 1 minute and throttle open more than 2%

51 - Check insertion of MEMCAL in socket and bent pins.

52 - Check that CALPAK is in place, fully inserted, and no bent pins.

53 - Car's alternator has produced >17.1 volts for over 2 seconds. Check charging system.

54 - Unknown error code. 

55 - ECU A to D error. Check ECU grounds, or excessive input voltage.