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OBD II Diagnostics...Simple As 1, 2, 3, 4Posted 6/7/2002By Kevin Caple
On-board diagnostics (OBD) II is making it easier than ever to diagnose cars. Well ... maybe not. But it is giving us information that can help pinpoint a problem with more accuracy and less time invested. One of the questions I am asked with great frequency is: “How do they count O2 sensors? Where is Bank 1, Sensor 3? Or ...Where is Bank 2, Sensor 2?” When thinking of O2 sensor positions, let's start with a single exhaust system in, say, a 1998 half-ton Chevrolet pickup truck with a 5.7L port fuel-injection engine. These trucks use four O2 sensors, but which is which? Bank 1 is always the bank where cylinder No. 1 is, or in this case, the driver's side (left) bank, and Bank 2 is the opposite (right) bank. After the point where the pipes meet, everything is Bank 1. Now, counting O2 sensors. One is located by the left manifold, which is Bank 1, Sensor 1 (scanner shows B1-S1). Moving back, the sensor in front of the catalytic converter after the Y-pipe is Bank 1, Sensor 2. After the Y, it's all Bank 1, but we still count the sensor in the manifold as 1. So now the sensor behind the catalytic converter is Bank 1, Sensor 3 (scanner shows B1-S3). On a dual exhaust system, the banks stay 1 and 2 all the way back. For example, on the same truck with dual exhaust ... Bank 1, Sensor 1 is in the left manifold; Bank 2, Sensor 1 is at the right manifold; Bank 1, Sensor 2 is behind the driver's side converter; and Bank 2, Sensor 2 is behind the passenger side converter. If automotive engineers decide to add more O2 sensors, they will continue the numbering in this fashion. Did you know the O2 sensors behind the catalytic converters have nothing to do with fuel delivery? Their only function is to tell the powertrain control module (PCM) how efficiently the converters are working. The PCM can set a service engine light to warn the driver of a low efficiency problem. A good catalytic converter can store a lot of oxygen. Since oxygen sensors read low voltage when they sense oxygen, and high voltage when there is a lack of oxygen, you should see a high number on the rear or post catalytic O2 sensors with a good converter because it has used up the oxygen cleaning the exhaust. As automotive manufacturers move ahead with tighter fuel control measures, expect to see an O2 sensor for each cylinder. When you think about it, wouldn't that make your diagnosis easier? You could actually see if a cylinder is running rich or lean due to an intake leak or a partially plugged injector. (Note: this is just to a certain point because if, for example, there was a misfire caused by an excessively rich or lean injector, it might show high oxygen due to no burn in the cylinder.) In the future, technicians may be able to do all their diagnosing from the driver's seat with a scan tool. Before you even open the hood, you could give the customer a parts and labor estimate (OK, now I sound like a test equipment salesman). All these technical innovations make today's cars and trucks run cleaner and more efficiently than ever before, and at the same time provide us with more information about the engine management systems. As always, the more parts there are, the more things can go wrong. So far, the O2 sensor systems have proven to be of little trouble and have been a great deal of help in diagnosing emission and driveability problems. In reality, nothing will take the place of good technicians. We need people with a thorough understanding of vehicle systems, with the aptitude and skill necessary to interpret data and relate it to vehicle performance and customer comments, as well as the mechanical skills to perform the repairs in a fast, efficient manner.
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Copyright (c) 1996-2009. Automotive Service Association®. All rights reserved.
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