How to Read OBD-I Codes Without a Scanner
OBD-I is not OBD-II
Anything 1995 and older usually speaks OBD-I. Each maker used its own connector, its own pinout, and its own list of codes. Most of them blink the check-engine light so you can count the code by hand. OBD-II arrived in 1996 with one 16-pin connector and one scanner that reads every car. Same warning light, different language. This site covers the older language.
Find the connector
On GM cars this is the ALDL, the Assembly Line Diagnostic Link. It usually sits under the dash on the driver side. The spot was never standardized, so it is not always in the same place from one model to the next. The GM codes page starts you in the same spot: under the dash, driver side.
Jump terminals A and B
Key off. Bridge terminals A and B on the ALDL connector with a paperclip or a short jumper wire. Turn the key on with the engine off. The check-engine light starts flashing. Count the flashes and write them down. The car is talking to you, one blink at a time.
Code 12 comes first
The first thing you will see is code 12. On GM that is one flash, a pause, then two flashes. It is not a fault. It means the computer is in diagnostic mode and ready. The real fault codes come after it. Each fault code flashes three times on GM, so you get three passes to write the number down.
Reading a code with two digits
Every code is a number, and the light flashes it one digit at a time. Watch the first group of flashes for the first digit. Then a pause. Then the second group for the second digit. A code 24 is two flashes, pause, four flashes. Keep your eye on the light. It does not wait around.
Clear the code and recheck
Fix the fault first, then erase the code. Clearing wipes the stored memory so the light can go out and the computer can watch the system fresh. On older GM cars that usually means pulling the ECM fuse or disconnecting the battery for a short spell. Then drive the car like normal. If the light comes back, the fault is still there. A code is a symptom, not a diagnosis. Never clear it and call the job done.
Other makes at a glance
GM and Ford are live on this site today. Chrysler, Toyota and Honda pages are in the works. Ford reads with the same paperclip idea but its own connector: jumper the self-test input to signal return under the hood, then read the key on engine off test and the running test. The Ford codes page carries the exact steps. Each other make reads its own way, so no terminal numbers are published for a make until its page is live. Check the home page for what is live.