Reading Fault Codes OBD2

Hi

My 'service engine' light is NOT on, but I do have a slight problem with my engines running.
My question is, would a fault code reader indicate any errors that would not neccessarily bring the light on? Or does every 'error' in the engine, energize the light.
I am just trying to prolong buying the fault code reader until I definitely will have a use for it!

Regards

Darren

we might be help in figuring out what the problem is by discribing the problem and mentioning what vehicle it is.

Code readers come in many flavors. The very basic ones do nothing but read codes, and if your light isn't on, odds are there are no codes in the computer, so those would do you no good.

Some of the higher end code readers can give you a data stream of what the computer is seeing/doing. But there is different levels of what data you will get. Some only tell you the bare basics which isn't terribly helpful. For ALL of the data, you need a manufacture-specific capable machine....and those are pretty spendy.

Even then, it doesn't always help you find the problem. Diagnosing problems still relies a lot on knowledge of what causes what, and good-ol' digging around the engine looking for problems in related areas

When I have a car I'm diagnosing though, I do usually start off by looking at the data stream, to see if there is any obvious issues that will give me a direction to look. The long term fuel trim is a great piece of info for how the engine management system is doing across the RPM range, as an example

Make sure the check engine light is working. Mine had three codes but the light bulb had blown :)

Its supposed to come on for a bit when you turn the key on to start the car for that reason; just to show you it works.