my ck lite comes on occasionaly but when I ck the over fopw tank is full but the gauge reads normal what can be causing this pls help it.s annoying

 

should be in 2nd gen section, and it means that you are getting air/bubbles trapped in your upper radiator for some reason. This could mean that you have a coolant leak somewhere, that the line to the overflow bottle is clogged/blocked, or a coolant seal is on the way out, pushing exhaust into the coolant (among other things). You really need to give a bit more info. How did you fill up the coolant system the last time? did you properly purge it?

 

topless can u please explain purging doe's the rad have a peccock drain

 

lots of this stuff is in the 2nd gen FAQ, and that section in general. On S4's, theres a little plastic screw on the side of the plastic neck from the top of the radiator to the upper coolant hose that you should take out when filling the cooling system from the cap on the thermostat neck, until coolant starts coming out. This helps get a lot of the air out of this local high point. Then put it back in, finish filling, and i always squeeze up and down the upper coolant hose to try to get more out, then start the car and rev it up for 30 sec or so with the upper coolant cap still off, check the level (and it usually falls cause of trapped air bubbles), top it off, rev it up again to check, put the cap on, and then fill the overflow bottle to where its recommended to be.