OBD1 is the on-board diagnostics fitted to cars and light trucks before OBD2 became mandatory on vehicles sold in the United States for the 1996 model year. If your vehicle is a 1995 or earlier US-market car, or an early import, this is the system it uses.
There is no universal OBD1 code list, and that is not an omission. Every manufacturer designed its own system: its own diagnostic connector, its own way of putting the car into diagnostic mode, and its own code numbers. Code 12 on one make means something entirely different on another. Always use the page for your manufacturer below – a code read off a generic list will send you after the wrong part.
How OBD1 codes are usually read
Most OBD1 systems report faults by flashing the check engine light rather than by talking to a scan tool. The general shape of the procedure is the same across most makes:
- Find the diagnostic connector. It is not the standardised 16-pin socket used by OBD2 – its shape and location vary by manufacturer.
- Put the system into diagnostic mode, usually by bridging two specific terminals in that connector with a jumper wire or paperclip, or on some makes by a key or pedal sequence.
- Turn the ignition on and count the flashes of the check engine light. The pattern – long and short pulses, with pauses between digits – spells out the code number.
- Look the number up on the page for your make.
Which two terminals, and where the connector sits, is specific to your manufacturer – and getting it wrong can mean shorting the wrong pins. Use the manufacturer page rather than a generic diagram.
A plug-in OBD2 scanner will generally not read an OBD1 system: the connector and the underlying protocol are both different. Adapters exist, but they are made for a particular manufacturer’s connector.
OBD1 or OBD2 – which do you need?
The two systems are listed separately on this site, and it is easy to land on the wrong one:
- 1995 and earlier (OBD1) – use the manufacturer links below. They lead to pages like Toyota and GM.
- 1996 and later (OBD2) – use the OBD2 code list, or the manufacturer-specific OBD2 pages such as Toyota OBD2 codes.
If you are unsure, check the emissions label under the bonnet: US vehicles built to the OBD2 standard say so on it.