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There are actually 4 separate DTCs for LFC (Light Flashout Code) 21.
B1938, B1935, B1933 and B1925.
They lead to 4 different pin point test but all refer to the passenger air bag circuits. It would be best to actually have the Air Bag Module (RCM) tested for the DTCs to isolate the proper pin point test to stop replacing good parts.
Get the codes and post them on this page. Some auto parts stores can test the RCM for codes for you for free.
See the chart below.
The ones I have been to will. Give them a call and ask first.
Down here I have Advanced Auto Parts, Pep Boys, Autozone and O'reillys.
Y can try disconnecting the battery for 10 minutes. But if the problem is still present, the air bag light will remain on. To erase the codes, you will need a scan tool or code reader to do it. Independent shops can do it also. You do not need a Ford scan tool to do it.