In 1996, On-Board Diagnostics (OBD) became the standard for cars and light trucks. Before then, vehicle diagnostics differed by brand. Ford vehicles used a system called Electronic Engine Control (EEC), and the 1991 Ford Tempo uses the fourth generation of that system (EEC-IV). An OBD-II scanner cannot be used on the 1991 Tempo, but there are code readers for EEC-IV vehicles. There is also a cheaper method that excludes scanners all together. Deciding which to use is a question of convenience. If you need to diagnose your Tempo often, buying a scanner is probably a good idea.
With Code Reader
Consult your code reader's user manual. How to use a code scanner is similar in spirit across brands and types, but the specific parts of the process do differ. Older code scanners, for example, need a lead attached to the Tempo's battery. Newer ones do not. Also, bookmark the pages that contain the EEC-IV trouble code definitions.2
Pop the Tempo's hood. You are looking for a two outlets. One is six-sided and is called the "Self Test Outlet." The other is a smaller one right next to it that has only one slot. This is the "Self Test Input." Connect the code reader to both of these. If your code reader has a lead that needs to be attached to the Tempo's battery for power, attach it to the positive terminal.3
Turn off the air conditioning. Engage the parking brake and shift the transmission into park or neutral. Warm up the engine to its average running temperature. Switch the engine off but leave the key in the ignition.4
Wait 10 seconds. Turn the key to power the electrical system, but do not start the engine.5
Return to the engine and switch the code reader on. If your reader features an LED screen, you can read the codes and key in commands. If you have a cheaper reader, you may have to count flashing lights (see the end of the Section 2 on how to interpret the flashes).
Without Code Reader
Engage the parking brake, shift into neutral or park, and turn off the air conditioning. Crank the engine and let it run for 10 minutes or until it reaches normal operating temperature. Turn the engine off.7
Pop the hood and look for the Tempo's six-sided self-test outlet and input. Connect the two with a jumper wire.8
Return to the driver's seat. Turn on the Tempo's electrical system but do not start the engine.9
Watch the check engine light. It will start flashing codes at you. EEC-IV codes are either two or three digits in length. There will be slight pauses between code numbers and longer pauses between the whole trouble code sequences. For example, code 15 will entail the check engine light flashing once, followed by a brief pause, then five more flashes.10
Consult the internet for code descriptions, since you will not have a code reader manual to consult. There are generic sites that archive EEC-IV codes and their descriptions (see Resources).