After 1996 all cars were made to hook up to an external error code reader. Before 1996 however, cars reported their errors from the computer themselves via a flashing code from the check engine light.
The dreaded check engine light comes on while you are driving only to signify that there is a problem. The specific error or failing part could be one of many different things that the computer senses for. You can bridge a set of wires in your car and send the cars computer into its error reporting mode so that you can find out whats wrong with your car.
Again, this instructable is only useful for people who have cars that report engine trouble via a flashing check engine light. This should be true for many cars made before 1996.
Here is a short video explaining the code deciphering process. For the full instructions check out the following steps.
Step 1: Find Your Vehicle's Diagnostic Plug
Just stick your head up under the dashboard on the passenger side and look around for a plug that isn't connected to anything.
If you don't have a Honda Accord then do a search online for where your plug is located and you will probably find some good leads.
Pull the diagnostic plug out from the plastic housing that is bundling it to the other set of wires under there (I think the second set of wires down there are sensors for the ABS brakes or the airbags).
Step 2: Bridge the Wires
Step 3: Watch the Flashing Light and Decipher the Code
Wait a second and keep your eye on the check engine light. You should see it start to light up in a series of flashes.
The light is blinking in a simple code of long and short. Long blinks mean "10" and short blinks mean "1".
My light was blinking once for a long time, and then twice for a short time. 10, 1 and 1 is error code 12. Code 12 indicates a problem with my EGR system. This is the system that sends some of the engine's exhaust into the cylinder to cool the combustion process and keep temps in the cylinders down. Having a broken EGR is relatively common on a high mileage car (I have 180,000 miles on mine) and it's a pretty easy fix. I was worried that I might have to pull over to the side of the road when I saw the light or get it to a mechanic ASAP, but now I know that it's not life threatening to the car and that it's OK if I get it looked at the next time I am at my mechanic.
You can find a list of error codes for the make and model of your car online by doing a simple search. The ones for Honda are included below.
Honda Check Engine Light Error Codes
1 Oxygen Sensor
3 Map Sensor
5 Map Sensor
6 Crank Angle
7 Throttle Angle
8 TDC Position
9 No. 1 Cylinder Position
10 Intake Air Temperature
12 Exhaust Gas Reiculation (EGR) System
13 Atmospheric Pressure
14 Electronic Air Control
15 Ignition Output Signal
16 Fuel Injector
17 Vehicle Speed Sensor
20 Electronic Load Detector
22 Vtec System Malfunction
23 Knock Sensor
41 o2 Sensor
43 Fuel Supply System
45 System Too Rich or Too Lean
48 Primary Heated o2 Sensor
54 Crankshaft Speed Fluctuation Sensor
61 Primary o2 Sensor Circuit
63 Secondary o2 Sensor Circuit
67 Catalyst Sytem
70 Automatic Transaxle
71-74 Random Misfire
80 Exhaust Gas Recirculation
86 Engine Coolant Temperature Circuit
90 Evaporative Emission Control System Leak Detected
91 Fuel Tank Pressure Circuit
92 Evaporative Emission Control System Insufficient