All Accord and Prelude models use an Engine Control Module/Powertrain Control Module (ECM/PCM) to control the fuel, ignition and emission systems of the engine. The ECM/PCM relies on a variety of sensors to evaluate the engine's operating condition. The ECM/PCM also has the ability to recognize when a sensor's value or input is beyond the normal operating range for that component.
If a sensor's wire is damaged, disconnected, or the sensor fails, the ECM/PCM receives an invalid signal for that sensor. The ECM/PCM has no idea what the cause of the problem is, however it does recognize that the signal is not within the acceptable operating range for that particular component. In order for the engine to run, the ECM/PCM will substitute a default value for the sensor that allows the engine to continue running, however the engine's performance and efficiency may be compromised. When this condition occurs, the ECM/PCM stores Diagnostic Trouble Codes (DTCs) into its fault memory.
If the condition is severe enough to cause potential damage to another component, the MALFUNCTION INDICATOR LAMP or the CHECK ENGINE LIGHT is turned on and the warning light remains on after the vehicle is started.
Diagnostic testing is done by checking the inputs and the outputs for the ECM/PCM control unit, and by accessing the fault memory of the ECM/PCM. The ECM used has on board diagnostic capabilities that are decoded by using the blink code method. All models covered are OBD-II compliant and the PCM fault memory is accessed by using a suitable OBD-II capable Data Scan Tool (DST), and the fault codes are displayed on the DST screen.
On all 1996-00 models, an OBD-II compliant scan tool must be used. To retrieve the Diagnostic Trouble Codes (DTCs). There are many manufacturers of these tools; a purchaser must be certain that the tool is proper for the intended use. A suitable good quality Data Scan Tool (DST) should come with comprehensive instructions on its proper use. Be sure to follow the instructions that came with the unit if they differ from what is provided in this guide.
The scan tool allows any stored codes to be read from the ECM/PCM memory. The tool also allows the operator to view the data being sent to the computer control module while the engine is running. This ability has obvious diagnostic advantages; as the use of the scan tool is frequently required for component testing. The scan tool makes collecting information easier; however, an operator familiar with the system must correctly interpret the data.
An example of the usefulness of the scan tool may be seen in the case of a temperature sensor, which has changed its electrical characteristics. The ECM/PCM is reacting to an apparently warmer engine (causing a drivability problem), but the sensor's voltage has not changed enough to set a fault code. Connecting the scan tool, the voltage signal being sent to the ECM/PCM may be viewed; and comparison to normal values or a known good vehicle reveals the problem quickly.
The most commonly required electrical diagnostic tool is the digital multimeter, allowing voltage, ohms (resistance) and amperage to be read by one instrument. The multimeter must be a high-impedance unit, with 10 megaohms of impedance in the voltmeter. This type of meter will not place an additional load on the circuit it is testing; which is extremely important in low voltage circuits. The multimeter must be of high quality in all respects. It should be handled carefully and protected from impact or damage. Replace batteries frequently in the unit.
Other necessary tools include an unpowered test light, and electrical leads capable of back-probing electrical terminals without damaging them. A vacuum pump/gauge is also needed for checking some sensors, solenoids and valves.