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On Board Diagnostics, version 2...It was not made manditory in the USA until 1996.

None, the 91 GMC was not ODBII

Because ODBII was 96 and newer.

On Board Diagnostics, version II (or 2)

You need an ODBI scan tool. A GM Tech 1 tool will probably work OK on this vehicle, as will a number of other similar tools.

Chunk of wire to jumper the pins is all you need. 91 isn't OBD2.

A scanner is not needed. Do a web search for how to do it and the flash count meanings.

Hi!

You don't. Your vehicle only supports OBD1 protocol. However, there is a bright spot. On most if not all GM vehicles with OBD1, you can short two terminals in the ALDL connector (look under the dash and online for directions) to get the codes sent to you by way of the "service engine" light. Each code (starting with "12" for "system OK") is relayed a single digit at a time, and repeated three times on top of that.

If your vehicle has a *factory installed* electric cooling fan, it may also run at the same time. This is not a problem.

That is because OBD2 only applies to 1996 and newer vehicles in the US...as well as a very few 94 and 95 model years (or so I'm told--never seen one). These won't work with your vehicle unless you use a hammer to insert the connector and then nothing will work at all.

On-Board Diagnostic 2.

William

Actually a bit of trivia. Code 12 is actually "missing engine speed signal" which indicates the engine is not running. The reason it appears (first) is that it is the lowest numbered DTC. It can happen while the engine is running, too. (Yes, I've seen and had to repair that problem.)

The best is WinALDL software for a PC, then in addition to codes, you can also get "live data". Get that here...

WinALDL

The above software is free, but you need an ALDL cable (ALDL to serial). Instructions for making your own or buy one here...

Then if you have a laptop computer without a "serial port", you need a USB to Serial cable and many sold in stores don't work right. The following works fine and not too expensive...

Parallax USB to Serial (RS-232) Adapter with Cable...