'92 300SE, as if I haven't already asked enough of these....

Today, get in the car after it has been in the sun for a while. Temp inside, I'd guess to be 85-90 degrees. A/C set at 75 both driver & passenger.

A/C comes on medium at best. I leave it there for about 3 or 4 minutes - no change at all. Cold air coming out center, left & right as expected, just no "urgency" to cool the cabin.

Press REST for 5+ seconds to activate sensor diagnostic mode - A/C immediately kicks into full cool-down mode. In-cabin temp sensor reports 85 degrees (confirming my original assumption.)

This behavior happens regularly - hot interior, no urgency of cooling from a capable system. Activate sensor diagnostic mode - BOOM - full cooling from then on.

What about the sensor diagnostic mode would cause the system to "wake up" and realize that more cooling is required?

Any suggestions? Last time I checked, which was recently, there weren't any stored trouble codes.

Thanks in advance.