My 1996 ford windstar had the checklight flashing when i drove it last. since, then i checked it with an obdII and it said po303 generic #3 cylinder misfire detected (red light). when running the engine for the obd and warming the vehicle the motor seemed ok - does this raise concern on part of the misfiring? other obd analysis was p1538 intake manifold runner control stuck open (beck 2), p0420 catalyst system efficiency below threshold (bank 1). the checklight is no longer on at the moment

SOURCE: I have a P0420 code catalyst efficiency below

it is a bad converter not o 2 sensor i would change the cat and both o 2 sensors

SOURCE: i have code 1538 on my check engine for 1998 ford windstar ,does

on the passenger side of the engine you will see 2 small metal rods at the top of the engine just below the intake manifold there are little white clips that hold the rods on these clips usually break and the rods come off

SOURCE: P0420 Catalyst System Efficiency Below Threshold

have you tried to replace the 02 sensors, make sure to use penetrating lubricant when removing them, you may need to apply heat as well to get them to come out without stripping out.

SOURCE: 1998 ford windstar 3.8 with code p1538. Is this a

LOOKING IN CODE BOOK.POSSIBLE CAUSE TO YOUR PROBLEM THE IMRC COULD BE STUCK OPEN.OR THE IMRC ACTUATOR IS DAMAGED OR HAS FAILED.ALSO THE IMRC MONITOR SIGNAL CIRCUIT COULD BE SHORTED TO POWER GROUND OR SHORTED TO SIGNAL GROUND RETURN.

SOURCE: received fault codes P0420 and P1296 catalyst

Not sure on the P1296 code, however in 95% of the cases the P0420 code means the catalytic converter is bad. See P0420 and P0420.com