Can't get gas cap back on 1999 e320

sounds like you need more antifreeze , make sure fluid is at proper line on overflow tank

yes
ignoring lights leads to complacency and a large repair account that could have been avoided if attention was paid in the first place
exhaust light codes are in the p1400 list and refer to the EGR valve system ( exhaust gas recirculation)
I would find an accredited service center that has experience on your make of car and have the fault codes read and the problem properly diagnosed
what you dealer stated about the gas cap is an EVAP issue and if he didn't read the codes first , he has done you a disservice and not attended to your concerns properly

If you are seeing air bubbles, then the cooling system still has air pockets in it. Loosen the overflow cap and leave it that way for a few days while you drive.Keep the heater turned on at the max heat level. Roll a window down if necessary. Check and top up the overflow bucket daily and top up as necessary. The bubbles should disappear after a few days, then you can tighten the cap. This should also solve your no heat problem. Once the bubbles are gone you can tighten the cap.

If the problem doesn't fix itself in a week, you should have the car looked at for a possible bad head gasket, which would force air into the cooling system. Hope this helps.

if the gas cap is installed properly(tightened),then you could have a code in the computer for the evap system that should be checked into.you will have to know what the code is to find what area of the system that needs to be checked out.

generally the light will quit in a weeks time
if not take it in to have the codes cleared to get the light off

chech seal on cap will normally reset after 15 min if cap was loose

Did you change the gas cap with a Honda gas cap? Tighten it at least three clicks?
Also, Honda recommends that you never top off the tank when getting gas (only go to the click-off of the pump; otherwise you can get back flow into the system that will cause the code).
Tightening or replacing the cap will not immediately cause the light to go off; it can take up to three days of driving for the light to reset.
To check if your readiness codes (emission system) are set, turn the ignition to ON (II), without starting the engine. The Malfunction Indicator Lamp will come on for 20 seconds. If it then goes off, the readiness codes are set. If it blinks 5 times, the readiness codes are not set.
You can, of course, try resetting your engine light to see if it will come back on (which it probably will). I believe the procedure is:
Turn off the engine.
Press and hold the Select/Reset knob (odometer) in the instrument panel, then turn the ignition switch to ON (II).
Hold the knob for approximately ten seconds until the indicator resets.

Hope this helps you.

first take the gas cap off for a few minutes and where the hole is to put the gas into the tank open that up with a screw driver for a few minutes and once the car starts put in a half a tank of gas in it and a bottle of STP gas treatment with fuel injector cleaner and a little more gas and if this helps you should be good and you may have to replace the fuel filter if it does not open it all the way be sure to put the gas cap back on after truck starts

Hi. this situation concerns a lose gas cap that you may have forgotten to tighten correctly during a recent fill up at your local gas station or, it could be that the original gas cap or replacement gas cap is not sealing the cap area correctly, preventing proper fuel pressure.

Check the gas cap to see if its tight and that it clicks when you turn it. make sure you turn until it clicks. this will ensure a secure fit.

Now, if you have just recently bought a new replacement cap, it may be the wrong fit and i would recommend purchasing the original replica form the dealership to ensure a tight fit.

Please rate and god bless:)

Most likely a gas cap or emissions issue, if you are sure the car is running fine. But you will have to have it scanned to know for sure.