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  You're driving down the road and the "Check Engine" light flickers on. You hope maybe it will go off, but it doesn't. You check around and find someone who says you can jumper the data link connector and see what the codes are. You go to a website that has the info or buy a book that tells you how to retrieve the trouble codes, and you follow the directions.

  Now you have the trouble code, what does it mean? The best phrase I heard to explain the trouble codes was by a very well versed trade school teacher, he said, “The trouble codes can tell you which street to go to, but not tell you which house.”

  For the sake of example, we will use a 93 Pontiac, with a 3.1 L engine that has set a Code 44, which is defined as Lean Exhaust Indication. A lot of people, including some mechanics, would run down and buy an oxygen sensor and replace it, hoping it solves their problem. Some will get lucky and it does solve the problem. Others are stuck, what did they do wrong? They replaced what the trouble code said, right? Wrong. Most codes descriptions state that it is a circuit problem.

  Now, you have the Code 44 and want to know what to do, where do you go from there?
Let's look at it from the viewpoint of a professional mechanic, what would he/she do?

• First thing would be to check for any obvious problems, anything that could be loose or disconnected.
• Next, time to pull some trouble codes.
• Now we have the code 44.
• So code 44 says O2 sensor circuit. NOT change the O2 sensor.
• Before we get too far, since we know it is an O2 code, we check for the obvious things again.
• These include;
  • Vacuum leaks
  • Disconnected, broken or frayed wires to the O2 sensor.
  • Poor fuel pressure from a bad pump or fuel pressure regulator.
  • Water in the fuel.
  • Exhaust leaks
• Now the mechanic would go to the diagnostic flowcharts and diagrams, do-it-yourselfers may want to check a reliable source, like Alldata, for manufacturer's diagnostic flowcharts and diagrams, which are the same ones as offered to the professional mechanic.

So, let’s see what the flowchart says.

Image used with permission  Copyright © 2001 ALLDATA LLC.

   You can see that it goes thru each step. A scanner is required to do this test, but you can see how you follow the map thru the diagnostics and eventually come up with the problem. Usually this one isn't the Oxygen sensor.

Image used with permission  Copyright © 2001 ALLDATA LLC.

  The flow chart will typically have an accompanying diagram. This one happens to be the wiring for the Oxygen sensor for the Code 44 flowchart, but that one is a relatively simple flowchart. Some of them can be quite extensive. As you can see by the flowchart and diagram, if you have the information of the trouble code, the flowchart and the diagram available to you, you can correctly diagnose the problem and repair or replace the right part the first time.

  Remember that often the trouble code doesn't mean to replace the sensor or component; it is telling you that there is a problem with that circuit. Once you have determined which circuit is the problem, then you have to go through that specific system to locate the problem. In this day and age, the diagnostic flowcharts and diagrams are becoming necessary tools to diagnose and repair the vehicles of today.

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