On classics, if the airstairs will not operate, check the striker pin (see photo below) at the bottom left of the door frame. Move it about and ensure it is vertical, this will often cure the problem. They have a tendency to freeze in position on long flights were the doors have got wet.
Caution: The handrails must be stowed before retraction. The use of standby system from either panel will bypass the handrail and lower-ladder safety circuits. Note that the NG has an red covered EMERG switch underneath the airstairs for emergency retraction, this also bypasses any safety circuits.
Maximum wind speed for airstair operation: 40kts.
Maximum wind speed for airstair extended: 60kts.
Airstairs should not be operated more frequently than 3 consecutive cycles of normal system operation within a 20 minute period.
Note that there have been at least 4 cases of children falling through the gaps in the rails of the airstairs FAA SAIB refers.
In the drive for self sufficiency, these were fitted to about 120 737-200's. They were much more complicated than forward airstairs as they folded in two places and took the door downwards with them. If you have ever considered the forward airstairs to be temperamental then you would not get on with aft airstairs.
There were several reports of inadvertent deployment and even two instances of them extending after take-off. Boeing say that after one of the in-flight deployments the crew landed with little control problem and apart from some scuff marks on the foot plates where they made contact with the runway, they were still in working order after the event!
737-3/4/500 Door showing latch fitting (above), striker pin (below) and 3 stop pin fittings
Underneath NG Airstairs - Notice the red guarded MAINT switch.