Did your check engine light ever come on and your mechanic told you that you needed a new catalytic converter, or some other expensive repair? There are a number of component failures that turn on the check engine light, but the most expensive to service is the catalytic converter. I'm going to explain how to read your car’s diagnostic codes and avoid an enormous repair bill.

The catalytic converter is part of the automobile exhaust system, placed between the engine and the muffler. It reduces the toxicity of emissions from internal combustion engines. First introduced in the U.S. in the mid-1970s to comply with EPA regulations for controlling auto exhaust, its functions are monitored today by the car's computer system with sensors on all important auto parts. If the sensor detects a problem, the check engine light will light up.

Replacing the catalytic converter can be pretty expensive for parts and labor combined. And if you are scheduled for your annual State Inspection, then you will be forced to pay for the repair, in order to pass the inspection. I once had this problem, and I knew several friends who also got stuck with an expensive repair bill when their check engine light came on.

The catalytic converter typically has a warranty exceeding the rest of the automobile's warranty length. (Exact details on this are in the EPA Environmental Fact Sheet. See the link in the table at the right). But if you fall outside of that warranty period by time or by mileage, and if you are in a state that implements vehicle emission inspection programs in your area, then you could be facing a large repair bill, especially if you are stuck with needing it repaired quickly, due to failing a state inspection.

Since I have a computer background, I am aware of an important issue. The only way the car's computer can know if a unit is failing is by a sensor reporting back to the computer. When the computer has a code stored, the check engine light is lit to let you know.  The computer registers a specific engine light code to indicate which sensor reported the problem. These codes are what are today known as the OBD I and OBD II codes, more on that in a moment.

Now, it is very possible that the sensors can fail, or get stuck, and temporarily report a bad code. If you have a universal catalytic converter, these are known to erroneously trip the sensor from time to time. Sometimes a sensor will break, and an honest mechanic will tell you that. Replacing a sensor is not that expensive. But what if you get stuck replacing an expensive catalytic converter?

Many times I have noticed the mechanic plugging in a code scanner under the dash to read the engine trouble codes from the car's computer. I thought how nice it would be to know what the computer has to say, before I go into the repair shop. So I decided to spend a little money on my very own Diagnostic Code Scanner. I did a few searches for these units and found really good prices and a large choice of models on Amazon.

Prices range from around $50 to the upper $200's...depending on the model. Each model offers different features. After a little review, I realized that I didn't need to buy the most expensive one.

If you have a 1996 or newer car, then the cheaper models of Diagnostic Code Scanners will do just fine. All cars since 1996 use the OBD II codes. The more expensive Code Scanners can read the older OBD I codes as well. That is simply not necessary to have, unless you have a really old car.

Personally, I purchased the Actron CP9135 AutoScanner Diagnostic Code Scanner with On Screen Definitions for OBDII Vehicles. This thing works great! It has a display screen that lets you flip through a menu of options to read the computer for any pending problem codes, read the status of the individual sensors, and even determine if the check engine light is on when it should be off, or visa versa.

There is no need to look up the codes in a table either, since it shows the description of the codes on the screen. It also lets you reset the check engine light by clearing the codes from the computer. Most diagnostic scanners offer similar features. Actron has a newer version (Model CP9145) now that also reads OBD I codes in older vehicles with an optional cable.

By reading the codes myself, I was able to see that my problem was indeed the catalytic converter. But there is no direct way to tell if it is really just a faulty sensor. So I cleared the computer and the check engine light went off, as expected. I drove around a few days to see if the check engine light would come back on. If it did, then I would be more certain that the catalytic converter needed to be replaced. But as I discovered, the sensor must have just given a misreading once, because the light never came back on. I reconnected the Scanner a few times to see how things were doing. For the first 100 miles since clearing the computer, it shows that he computer is not ready. This is because it needs to wait throughout this period to determine if an error status is reported again.

In case you are thinking that I blew the light by turning it off, that is definitely not the case. First of all, the Diagnostic Code Scanner checks the condition of the check engine light as well. And secondly, when you turn on the ignition without starting the engine, all the panel lights light up as a test. So I can see that they all are working.

I had saved a few friends already from this potentially expensive fate, with my Diagnostic Code Scanner.

Conclusion: Getting your own Diagnostic Code Scanner can save you from a mechanic telling you that you have a major problem which may just be a result of a faulty or tripped sensor.

New Safety Monitoring Standards - Tire Pressure Monitoring

I added this info after researching the answer to a question in the comments below.

In 2008 the National Highway Traffic Safety Administration introduced a new requirement that all new cars and light trucks must have a Tire Pressure Monitoring System, known as TPMS. These newer cars and trucks have a TPMS warning light in the dash, which will come on if the pressure on any of the tires is bellow 25% of the recommended pressure. A flashing or blinking TPMS light means that one of the TPMS sensors is malfunctioning.

The TPMS codes are not included in the ODB I or II codes and will not be recognized by an OBD scanner.  A company named Autel makes a scanner specifically for the purpose of reading the TPMS codes, as shown at the right. This scanner displays diagnostics results of Sensor ID, Tire Pressure, Temperature, Battery Condition and OE part number. Unlike the method of reading the OBD codes, no physical connection is required to read the status of the TPMS. It reads the data via wireless signals.