I have heard that when the check engine light is on that your car runs in like a "safe mode" kind of thing... is this true?

 

No, not really.

It depends on why the check engine light is on. A bad knock sensor will cause that to happen but a I can think of countless other codes that would not affect performace at all. The CEL light itself means nothing, it is just a light. It is the problem behind that light that matters.

 

how bout a bad coolant temp sensor. I know taht should keep fans on but would that also stop the car from revving past 2k rpm? just so it doesnt overheat?

 

what if you reset the ecu after the knocking code comes up and the light doesnt come back on?

 

It depends...

If the knock sensor code appears it is because the knock sensor is not working correctly. When the knock sensor is not working correctly the ECU will go in "safe mode" as you call it. Without a correctly working knock sensor the ECU will no know when too much timing advance causes detonation. So if the ECU cannot rely on the knock sensor to tell it when to pull a little bit of timing it will just constantly retard the timing to be "safe".

The CEL does not always come on the instant there is a problem, there can be a delay. I once had a terrible misfire and did not get a misfire code for a couple days.

So your knock sensor code poped up for a reason. One of which is that it is not working properly. You can clear the code, but that does not mean the ECU will automatically go back to full timing mode. It just mean the CEL is no longer on. The ECU may still read a improper working knock sensor and constantly retard timing.....but the CEL will eventually come back. It may be a 100 miles or so but it will come back.

I would always clear a code and see if it come back before fixing the indicated problem.

But you could just avoid all the guessing and check it with a ohm meter and know for sure.