To read the codes on an OBD-I system, all you need is a paper-clip
Look in the engine bay and you'll see a small black box next to the left hand side strut tower that says 'Diagnosis' on it. Get a paper-clip or a small piece of wire and put one end into the port labelled TE1 and the other end into the port labelled E1 (the labels are found on the underside of the diagnosis box cover, much like a fuse-box).
Turn the key to the 'ON' position, but don't start the engine. If your connections are correct then the check engine light should start flashing. Keep note of how many times it flashes as this is the code it is giving you. For example, if the code was 35 it would flash 3-times, have a short pause, then flash 5 more times, then have a long pause. If it repeats the same code after the long pause then that means it's the only fault detected, but if it flashes for a different period of time then that means there is more than one fault. Take note of any codes it's giving you.
The meaning of the codes are as follows:
12 = RPM signal (no RPM signal to ECU within several seconds after engine is cranked)
13 = RPM signal (no RPM signal to ECU with engine speed above 1,500rpm)
14 = Ignition signal (no ignition signal to ECU)
16 = ECU control signal
21 = Main oxygen sensor
22 = Coolant temperature sensor
24 = Intake air temperature sensor
25 = Oxygen sensor or circuit (lean)
26 = Oxygen sensor or circuit (rich)
27 = Sub-oxygen sensor
31 = MAP sensor
41 = Throttle position sensor
42 = Vehicle speed sensor
43 = Starter signal
51 = Switch condition signal
52 = Knock sensor signal
53 = EGR system
1993 Toyota Corolla
Linear Mode
