A stop light or idle stall often means the engine is not idling fast enough (idle speed too low), or the engine is being lugged down by a load on it created by the air conditioning compressor and/or alternator. It could also mean the fuel mixture is too rich or too lean, causing the engine to run poorly. Possible causes that may contribute to this kind of stalling include:
* A Bad A/C compressor. If the A/C compressor is binding up, possibly due to a lack of lubrication, internal wear or an over-charged A/C system (too much refrigerant), it may be lugging down the engine when it is engaged. If the problem only occurs when the A/C is on, there is an issue with the compressor.
* Unusually high electrical load on the charging system. If the battery is run down and the alternator is working hard to recharge it, the increases load on the engine may pull down the idle rpm to the point where it causes the engine to stall. Check the battery state of charge to see if the battery is run down or failing. If the battery is low, use a battery charger to recharge it, or drive at highways speeds for half an hour or so. If the battery is failing and is not holding a charge, time to buy a new battery.
NOTE: Low voltage can adversely affect the operation of the ignition system and fuel injectors, causing stalling and misfiring. A good charging system should produce about 13.5 to 14.5 volts at idle.