The electronic control module (ECM) on your Honda controls ignition timing, fuel injector timing, the length of time the injectors are open and several other things. It gets signals from sensors on the engine, in the form of voltage, then matches the signals to predetermined programming within the computer. It is then able to send a signal to other sensors and components on the vehicle to control those components. The ECM cannot diagnose itself. It can only send a code and turn the engine light on when the sensors' signals are out of range.
Plug a code scanner into the data link port. On most Hondas, the port is under the driver's side dash, just to the left of the steering column. Turn the key to the "On" position, then press the "Read" button on the scanner.
Write down the codes and compare them to the code sheet that comes with the scanner. If there are too many codes -- you know that many things cannot be wrong with the vehicle -- or if you have erroneous codes, then the ECM is suspect. An example of an erroneous code would be if the coolant temperature sensor code shows up, but you know it's working. Also, codes that do not exist for your year and model Honda could show up if the computer is not working properly.
Unplug the code scanner and turn the key to the "Off" position. Disconnect the negative battery cable, using the appropriate wrench. Locate the ECM, usually under the footrest on the passenger side floor. It may also be in the kick panel on the passenger side.
Remove the kick panel or carpet, depending on the location of the ECM. Ground yourself, as even a small shock of static electricity can damage the computer. Remove the ECM from its bracket. Using the appropriate socket, loosen the bolts that hold the wiring harness connectors into the back of the computer. Pull the connectors straight off the computer, ensuring that you don't bend the pins.
Remove the cover on the computer to gain access to the PROM chip. Inspect the inside of the computer. If the computer smells like dead fish, it is highly suspect. If you see burnt areas near the solder joints, the computer is bad.
