1999 mustang, does not like hot days, help!
 

We got a '99 mustang from a private owner about a year ago. We have a few problems that we cannot narrow down.


1. AC blows hot at idle after reaching normal operating temp. Once car is moving again, ac blows ice cold.
2. When outside temps are over 90 degrees, runs fine for 20 to 30 mins, then begins running rough, and stalls. Will restart after just a few mins, but goes back to same within 5 mins. CEL is not on.

The last issue is the biggest problem at the moment. I am the sole driver, and when I took my boyfriend out to try to get the problem to reproduce for him, of course it didn't happen. We did notice the pressure relief valve on the radiator cap was open, and closed it before we took her out. We checked the OBDII codes and got
P0161- heated O2 sensor circuit malfunction
P0141- heated O2 sensor circuit malfunction (bank 1, sensor 2)
P1309- AICE chip fault
Since the P1309 code could be so many different things, is there a good place to start?

A new fuel filter should be put on every year at least.
Both after cat O2 sensors may be bad but locating the cause is important. You gave a great description of the symptoms but I'm afraid you may need to have a Mustang shop run a diagnostic test on it.
In the mean time it would be a good idea to get the car ready for diagnostic testing by replacing the fuel filter, clean the MAF sensor, and Seafoam the gas tank and use another can pulled up a vacuum line to clean the upper intake manifold. They appreciate a car that has dependable "tune up" parts installed also, ie. air filter, fuel filter, spark plugs, pcv valve, ect....
The AC system is probably a little low on charge. If it is cycling on and off & needs a charge, remember that it also indicates there is a leak & it will need fixed too. Make sure the radiator coolant is fresh and the overflow reservoir is filled to the top line.