[h=Definition]1[/h]
The ECU or Engine Control Unit is used to control and diagnose engine control systems

[h=History]1[/h]
In the mid to late 80’s Honda introduced Programmed Fuel Injection know as PGM-FI to its line of on-road vehicles motorcycles and Formula 1 engine packages. The PGM-FI system Honda introduced to it’s motor vehicles consisted on an ECU, sensors that monitored engine temperature , air temperature, manifold pressure , barometric air pressure, oxygen content in exhaust gasses, engine rotation and vehicle speed to control engine fuel mixtures via injectors and engine idle speed via an Electronic Air Control Valve.

Early PGM-FI systems only controlled fuel mixtures and the ignition system was controlled by a conventional vacuum advanced equipped distributor with an external coil and ignition igniter module. The ECU used in this early PGM-FI system had basic onboard diagnostics where faults with the systems input sensors would be stored on the onboard memory and commence blinking an onboard LED viewable via a small window in the ECU housing with the relevant fault code to the system that had triggered the fault, the engine lamp on the cluster would also be illuminated to alert the driver of a fault. Some internet groups refer to this system as pre OBDO.

[h=OBD0]2[/h]
In the late 80’s ( 1988 onwards ) Honda introduced a PGM-FI system with an ECU that was capable of controlling both fuel mixtures as well as the ignition system. The ECU connectors are the same as the Pre OBDO however they have different pin orientation. This system is known as OBDO. The OBDO system supported the same diagnostics system as the pre OBDO system mentioned above however the ECU featured a secondary processor that was used when a critical fault occurred. With the event of a critical fault with a sensor or the ECU itself the ECU would failover to this secondary processor and run the engine with limited RPM and a richer fuel mixture to protect the engine. The fuel and timing maps in this mode would come from the backup processor and not the main ROM space from the primary processor, this mode is known as Limp Mode. More sensors where introduced to aid the Ignition system and the OBDO era saw the introduction of Honda’s Variable Valve Timing and Electronic Lift Control (VTEC ) system. The VTEC equipped vehicles had an additional oil pressure sensor to monitor VTEC engagement and a VTEC spool solenoid to allow the mechanical engagement of the Variable valve timing system. VTEC capable ECU’s have four map tables, they consist of Low Speed Ignition, Low Speed Fuel, High Speed Ignition and High Speed Fuel tables.

[h=OBD1]2[/h]
The 90’s era saw the introduction of Honda’s OBDI system. The OBDI ECU shared the same principle in operation as the OBDO ECU with more sophisticated functionality and diagnostics available utilizing Honda’s PGM-FI Tester that would connect to the ECU via the 3 pin Data Link Connector (DLC)

[h=CEL Codes]1[/h]
CEL or Check Engine Light codes can be used to diagnose faulty engine management system.

The following is a list of common Honda CEL codes.

1 O2A - Oxygen sensor #1
2 O2B - Oxygen sensor #2
3 MAP - manifold absolute pressure sensor
4 CKP - crank position sensor
5 MAP - manifold absolute pressure sensor
6 ECT - water temperature sensor
7 TPS - throttle position sensor
8 TDC - top dead centre sensor
9 CYP - cylinder sensor
10 IAT - intake air temperature sensor
12 EGR - exhaust gas recirculation lift valve
13 BARO - atmospheric pressure sensor
14 IAC (EACV) - idle air control valve
15 Ignition output signal
16 Fuel injectors
17 VSS - speed sensor
19 Automatic transmission lockup control valve
20 Electrical load detector
21 VTEC spool solenoid valve
22 VTEC pressure valve
23 Knock sensor
30 Automatic transmission A signal
31 Automatic transmission B signal
36 traction control found on JDM ecu's
41 Primary oxygen sensor heater
43 Fuel supply system
45 Fuel system too rich or lean
48 LAF - lean air fuel sensor
54 CKF - crank fluctuation sensor
58 TDC sensor #2
61 Primary oxygen sensor
63 Secondary oxygen sensor
65 Secondary oxygen sensor heater
71 random misfire cylinder 1
72 random misfire cylinder 2
73 random misfire cylinder 3
74 random misfire cylinder 4

Further information on how to interrogate an ECU can be found here:
http://www.ozhonda.com/forum/showthr...K-ECU-for-j00-!