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10094 The OBD II Story

Created on 2011-07-29 by IPD Staff

Automotive emissions control was first introduced in the early 70s. In the following years it was refined and expanded always with the idea of reducing the type and quantity of unwanted emissions. As technology became better, computers began to be used in more and more models to further refine the emission control system. It was a logical next step to incorporate some form of self-diagnostics into this system. The first generation of this diagnostic ability was introduced as early as 1987 in some vehicles and consisted of a small black box under the hood. A plug in wire was inserted in one of 6 holes and pushing a button would cause the computer to read out codes by flashing a red LED. This was a simple system and although it was better than nothing, it left a lot of guesswork in the diagnostic prosses.

OBD II was required to be incorporated in all automobiles beginning in 1996. Volvo added the system to most of it US models in 1995. OBDI preceded OBDII but was not as widely used and was only incorporated into a few model years from 1993 to about 1994. OBD II consists of a standardized plug located in the passenger compartment of the car. The engine control computer is equipped with a recording system that logs codes for components that it finds are out of tolerance and can communicate this information through the plug.

Here's how it works:

Let’s take the example of a throttle position sensor. This is a small variable resistor (potentiometer) that is fastened to the engine throttle. This is the part in the engine that moves when you step on the gas pedal. This is an important piece of information that the engine control unit needs to know to operate the engine. The throttle position sensor receives voltage from the engine control unit and modifies this voltage based on the position of the throttle (your foot). This changed voltage is read by the control unit as the position of the throttle. If the control unit reads this voltage, and finds it to be out of the range it expects to see, it will set a code.

Here’s an important point. Like any computer function, people designed it. Someone along the way set up a parameter that they felt should be the operating range of the throttle position sensor. This is the criteria that is used to decide if the throttle position sensor is out of specification. Like anything of this sort it turns out that codes are not the ‘silver bullet’ to 100% accurate repair. There can be situations where a code is set that may not really be true for the component it is reporting. What needs to be understood is that codes should never be taken as the absolute end of the diagnosis path. In this case the inaccurate voltage might be the result of corrosion on the plug that is connected to the throttle position sensor. It could be caused by the plug itself being loose. There will almost always be some other possibilities other than the failure of the item itself.

Most of the units that are used to in the engine management system are tested in this way. When a code is recorded (set) in the system that has an effect on emission, the control unit will turn on the check engine lamp. This is the little light on the instrument panel that you should see every time you start the engine. It should come on to confirm that the light bulb is OK and then go out. If it comes on while you are driving this means that the system thinks something is far enough out of perimeters that it may raise the emission level of the car. This actually an EPA required system and should never be disconnected or tampered with.

The difficult part of all this is only a code reader or a scanner can be used to retrieve codes and see what they are. The good news is that most of the codes are generic and all cars with the system will use the same code for the same component. These are often referred to as the P0 codes. They take the form of the letter P followed by 4 digits. P stands for powertrain, which relates to things that make the car move. The automatic transmission is also included in this designation. The generic code will always start with P0 followed by three digits. Codes that are unique to a particular manufacture will start with P1xxx. It is not uncommon for any given component to have several fault codes assigned to it. In the case of the throttle position sensor it might have a code for open circuit, signal to high or signal to low.

Another interesting thing about these codes is they may have nothing at all to do with actually running the engine. The fuel tank in all modern cars has an elaborate system to check for vapor leaks that might cause fuel vapor to escape into the atmosphere. If a problem is detected in this system the check engine lamp will also be illuminated. As you imagine this system has no direct effect on how the engine performs, none the less the check engine lamp will be iluminated. Many times codes in this area can be triggered by the gas cap being loose. If codes like this are set, and the check engine lamp has turned on, the lamp may go off by itself. In the case of the loose gas cap if its reinstalled properly and the system rechecks itself, and no faults are found, the light will be turned off with no action on your part. This self-canceling feature may apply to other components as well. So don’t be surprised to see the check engine lamp come on for a few days and then go off.

Part of the EPA’s logic in requiring the OBD II system was to allow a quick form of emission testing. The system has a feature built into it that allows a specific type of scanner to look over the system and determine whether or not the car is not producing excessive emissions. Several states have adopted this as method for required periodic emission inspection. There is a potential for a problem in this area that is useful to be aware of. In order for an emission inspection like this to be effecive, the engine management system must operate in all the various modes that it might encounter. This is to be sure that the all of the components can operate throughout their possible ranges. This requires that the car be driven at various speeds and conditions. When all of these driving criteria are met the system has successfully checked out all its components. This is referred to as a “trip”, or EPA driving cycle. If a “trip” has not been completed all of the components may not have been checked and the emission test is not completed. As each of these driving condition are met the system will set what are referred to as “flags”. This means that with this type of emission testing it may happen that the OBD II test is failed even though no faults are found and no trouble codes are set. There are some Volvo models that are particularly prone to this symptom. Its is important to note that once the required driving cycle is completed it does not have to be done again. The trip completion is stored in the engine management’s memory unless “THE BATTERY IS DISCONNECTED”. This will erase the driving cycle and make it necessary to complete the “trip” all over again.If you have had problems in this area you should avoid disconnecting the battery if at all possible. With a little care it is even possible to replace the battery without loosing power.

Many scan tools have the capability of reading the output of some of the sensors and displaying the data in real time. Information like engine temperature, RPM, boost pressure, throttle position. Some models will even calculate instantaious fuel mileage. So what should these numbers be? The best way to answer this is to observe this data when the engine is performing normally. Get an idea of what is normal so it becomes obvious when it’s not. This can be real useful information when a problem occurs.


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