Diagnostic Aids

Code 28 will set if PCM detects one of 2 "illegal" fluid pressure switch assembly combinations. See VALID FLUID PRESSURE SWITCH ASSEMBLY COMBINATION table below for various combinations. Check pass-thru connector for good contact.

VALID FLUID PRESSURE SWITCH ASSEMBLY COMBINATION

Value	A	B	C
Park	Off	On	Off
Reverse	On	On	Off
Neutral	OFF	ON	OFF
4th	Off	On	On
3rd	Off	Off	On
2nd	Off	Off	Off
1st	On	Off	Off
Illegal	On	Off	On
Illegal	On	On	On

Fig 1: CODE 28, Schematic, FPS Assembly Fault (Astro, Safari & C & K Series)

Courtesy of GENERAL MOTORS CORP.

Fig 2: CODE 28, Schematic, FPS Assembly Fault ("G" Series)

Courtesy of GENERAL MOTORS CORP.

Fig 3: CODE 28, Flowchart, Fluid Pressure Switch Assembly Fault

Courtesy of GENERAL MOTORS CORP.