The Reason Why we should use Obd II Codes

Automotive diagnostics has been the basis on troubleshooting vehicle problems since electronic emission controls have been incorporated by manufacturers in vehicles. Electronic devices such as sensors and engine control units (ECU) are part of engine operating systems. The OBD (On Board Diagnostics) is actually done by the ECU. There are values programmed in the ECU to detect optimum engine performance. It is designed to compensate for changes in engines operating processes. An example here is how the ECU compensates for engine operation when starting cold. The ECU makes the car "feel" like warm when it starts from cold. When there is something wrong with the sensors responsible for this type of operation, the ECU will record that error. That error is determined by the car's engine control unit from a list or bank of codes programmed into it. When an engine is diagnosed that will be the time the ECU will give the information through a code. The code is what is referred to as the OBD code.

The OBD II codes and automotive repair manuals are being discussed here because it is now generally for all vehicles manufactured since 1996 to present. Its ancestor, the OBD I, was not actually a standard and was not referred to as OBD, until the standard requirements of OBD II came out. The codes are actually called Diagnostic Trouble Codes (DTC). The DTC for different brands of vehicles vary as they are constructed differently. The basic codes are the P (Powertrain) codes. The codes for this are like from P0001 to P0099 for Fuel/Air Metering and Auxiliary Emission Controls. The first character "P" identifies the system related to the trouble code. The second digit "0" identifies whether the code is a generic code or a manufacturer specific code. The same for all OBD-II/obdii code reader equipped vehicles. The third digit denotes the type of sub-system that pertains to the code such as: 1 = Emission Management (Fuel or Air); 2 = Injector Circuit (Fuel or Air); 3 = Ignition or Misfire; 4 = Emission Control; 5 = Vehicle Speed & Idle Control; 6 = Computer & Output Circuit; 7 = Transmission ;8 = Transmission. The digits 9 and 0 are reserved by the Society of Automotive Engineers (SAE). The fourth and fifth digits denote the type of defect or malfunction.

So, let's say that the DTC code that shows is P0171. This means the system fuel-to-air mixture is too lean. If the code is P0271 it will mean that cylinder 4 - Injector Circuit High. Or, if the code is P0371, - Timing Reference High Resolution Signal "A" Too Many Pulses. Please note that the third digits are different. When you refer to the above on what the third digit denotes: 1 - is for emission management; 2 - is Injector Circuit and 3 - Ignition or Misfire. Now, relating the DTC codes shown as examples here, to the third digit identification, it immediately points out to the system that would be malfunctioning or is defective. Knowing what these codes mean now, having a diagnostic tool will make you better understand the codes that show.