Check Engine: On-Board Diagnostics (OBD)

I told my friend Tom that he's now my official automotive consultant. Acting on another tip from him, when my "Check Engine" light came on I went to Advance Auto Parts in Columbia, MD to borrow their car code reader to diagnose the problem.


Car Codes
Advance lets you use their car code reader if you leave your driver's license at the counter. The unit they have is the Actron CP9175 OBD-II AutoScanner, which costs about $110 on Amazon.com.

This unit lets you read the industry-standard On-Board Diagnostics II (OBD II) codes from your car's integrated computer. OBD trouble codes are used to diagnose car problems, most notoriously the "Check Engine" light which covers such a wide range of possible causes that it's impossible to tell otherwise what has triggered it.


The last time the Check Engine light came on, it was because I didn't tighten the gas cap enough after filling up my tank, but I didn't think that was the case this time. So I wanted to read out the trouble code to assess whether this was a serious issue that I needed to deal with immediately.

You'd think it'd be easy today for the manufacturers to provide the codes and a description directly, since many cars have digital screens, but they still rely on the antiquated indicator lights. But at least OBD-II is available on almost every car shipped since 1996.

Using The Car Code Reader
The reader plugs into the connector on your car, usually under the driver's side dash (near the fuse box, in my case). The car key needs to be in the "on" position.




Once plugged in and turned on, I selected "Read Codes" and the device went to work. In about a minute it returned a result, the error code P0420.



I used the reader to clear the code (though it came back on shortly afterwards) and then turned to the Internet to interpret it, since Advance wasn't able to tell me.


Trouble Code P0420
It quickly became clear that my problem is either an oxygen sensor or my catalytic converter. P0420 comes on when the oxygen level exiting the catalytic converter is too close to the same level as the dirty exhaust entering (indicating that the catalytic converter isn't cleaning the exhaust well enough).

Because this is primarily an emissions issue rather than something that will damage the engine, I didn't take it into the shop immediately as my schedule was very heavy. About a week later the light went off on its own when the weather warmed considerably (about 15-20 degrees).

My theory at this point is that my catalytic converter is probably the culprit, since it has 150,000 miles on it and my engine has been burning oil for the past year, and I'd hazard the guess that the converter's efficiency dropped below the acceptable threshold during our recent spell of extremely cold weather. 

Replacing a catalytic converter can be expensive so I'll conduct research before getting a replacement, especially since they usually rust onto the exhaust pipes after so many years in service (making them difficult to remove).