Here's how it works for my 91 Toyota Four-Runner:

Check engine light procedure and codes

The ECU detects malfunctions or abnormalities in the sensor network and lights a check engine warning light on the Instrument panel. At the same time the trouble is identified and a diagnostic code is recorded by the ECU. The diagnostic code can read by the number of blinks of the check engine warning light when terminals T and E1 are bridged within the diagnostics port.

Instructions for checking the ‘Check engine warning light’
1.   Locate the diagnostics socket in the engine , found on the right hand side of engine.
2.   Bridge terminals TE1 and E1 with a thin piece of wire.

3.   The "Check engine warning light’ on the dashboard will start to flash with any stored error codes
4    The number and consistency of the flashed indicates the error code.

Example : Error 24 - Intake Air Temp. Sensor fault

FLASH…FLASH………….….FLASH…FLASH….FLASH….FLASH

Each number is separated by a pause. Therefore if the light flashes twice then pauses, then flashed four times , the code 24 is produced. The code is continuously repeated, and there may be more than one error code stored.

Once the fault is diagnosed and cured, it is a good idea to erase the errors codes from the ECU by disconnecting the Battery Negative cable.

1	Normal Condition.
2	Air Flow Meter signal.
3	Ignition signal.
4	Engine Coolant Temperature Sensor signal.
5	Oxygen Sensor.
6	RPM signal (Crank Angle Pulse).
7	Throttle Position Sensor signal.
8	Intake Air Temperature Sensor signal.
9	Vehicle Speed Sensor signal.
10	Starter signal. (STA signal)
11	Switch signal.
11	ECU/ECM.
12	Knock Control Sensor signal.
12	RPM signal.
13	Knock Control CPU (ECM).
13	RPM signal.
14	Turbocharger Pressure.
14	Ignition signal.
21	Oxygen Sensor.
22	Engine Coolant Temperature Sensor signal.
23	Intake Air Temperature Sensor signal.
24	Intake Air Temperature Sensor signal.
25	Air-Fuel Ratio Lean.
26	Air-Fuel Ratio Rich.
27	Sub Oxygen Sensor signal.
28	No. 2 Oxygen Sensor signal.
31	Air Flow Meter signal (Vacuum Sensor signal).
32	Air Flow Meter signal.
34	Turbocharger Pressure signal.
35	Turbocharger Pressure Sensor signal.
35	HAC Sensor signal.
41	Throttle Position Sensor signal.
42	Vehicle Speed Sensor signal.
43	Starter signal.
51	Switch signal.
52	Knock Sensor signal.
53	Knock Sensor signal.
54	Inter-cooler ECM signal.
71	EGR System.
72	Fuel Cut Solenoid signal.
78	Fuel Pump Control signal.
81	TCM Communication.
83	TCM Communication.
84	TCM Communication.
85	TCM Communication.