I have mixed feelings on the CEL. While it does point you in the right direction on what the problem is most of the time, there are times that what the CEL is telling you can be the problem of something else that you wouldn't think is related. Case in point.
A few years ago one of my 92 Cavaliers(2.2L 4 cyl) set off the CEL and the code was for the EGR system. The EGR valve on this engine is a vacumm operated diaphram model. Long story short after replacing the EGR, EGR valve solenoid, checked all vacuum lines and any thing else I thought that could be related, I took it to an independent shop and then the dealer before it was fixed. The problem the entire time was a bad ground on the engine and this EGR valve is not electronically operated. Granted this is an OBD I system, but I have run into similiar situations on OBD II systems.
I also used to have a 93 S-10 4x4 with the 4.3L that I bought new. At about 45,000 miles, the CEL started coming on occasionally. It would usually come on within the first few miles of driving and then go out. The code it set was for the Knock sensor circuit. I replaced the knock sensor and the connector with no results. I took it to a couple of different chevy dealers with no one being able to fix it. I decided to sell it not long after and never did get it fixed. I sold the truck because I needed a bigger one but as anal as I am about how vehicles run and their maintainence, the CEL coming on was driving me crazy and it was a relief when I got rid of it because I didn't have to deal with it anymore.
Just because you know the code and what it means, does not mean you will be able to fix it. You need to understand how the systems operate that the CEL is referring to and to also understand how to read and interprete the diagnostic flow charts that accompany most trouble codes in the service info.
I do have an AutoXray 6000 scanner which is a decent DIY scanner and has helped me in the past. I also own factory service manuals for all of my vehicles which also help but there is still a lot of this stuff I don't understand and don't have the correct equipment to diagnose. I would love to have a Tech II scanner for my GM vehicles but the cost is just too much for me to justify as a DIY'er.
The other disadvantage that I have seen with the addition of the CEL is that it seems like it has made mechanics rely way to much on a computer to fix something rather than really take the time out to understand what really makes these new vehicles run.
I have had quite a few mechanics, some even from dealerships, tell me that if the CEL is not on, than as far as they are concerned, nothing is wrong with the vehicle. I really don't like that attitude but maybe that is what they are taught these days.
In this day and age as others have pointed out, it is crazy not to have at least a basic scanner to at least give you an idea of what may be wrong and then you can search the internet to give you some ideas on what might be causing that particular code to come up.
