We bought my son a '94 F150 XL. The SRS lamp flashes out a 2-1 code.
I've seen this described as a "safing" sensor (what is that) or a ground issue with the module.
All the lists I've found are for later years.
Can anyone clarify what code 21 is and how to correct?

 

Are you sure it is code 21?

CODE COMPONENT/FAULT
12 Low Battery voltage
13 Air Bag Circuit Shorted to Ground
14 Front Air Bag Sensor and Bracket Circuit Shorted to Ground
21 Rear Air Bag Sensor and Bracket Not Mounted to Vehicle Properly
22 Rear Air Bag Sensor and Bracket Output Circuit Shorted to Battery Voltage
23 Rear Air Bag Sensor and Bracket Circuit Input Feed/Return Circuit Open
24 Rear Air Bag Sensor and Bracket Output Feed/Return Circuit Open
32 Driver Side Air Bag Circuit High Resistance or Open
34 Driver Side Air Bag Circuit Low Resistance or Shorted
41 RH Front Air Bag Sensor and Bracket Feed/Return Circuit Open
42 LH Front Air Bag Sensor and Bracket Feed/Return Circuit Open
44 RH Front Air Bag Sensor and Bracket Not Mounted to Vehicle Properly
45 LH Front Air Bag Sensor and Bracket Not Mounted to Vehicle Properly
51 Air Bag Diagnostic Monitor Internal Thermal Fuse - Fuse Blown Due to Intermittent Short to Ground
52 Backup Power Supply - Voltage Boost Fault
53 Front Air Bag Sensor and Bracket Circuits Resistance to Ground or Air Bag Diagnostic Monitor Fault
- Rapid Continuous Flashing of Air Bag Indicator (No Fault Code) All Primary Air Bag Sensors Disconnected
- No Air Bag Indicator - Inoperative Indicator Circuit or No Battery Positive Voltage to Air Bag Diagnostic Monitor
- Continuous Air Bag Indicator - Air Bag Diagnostic Monitor Disconnected or Inoperative

Here are the airbag sensor location.................

Air Bag System Component Location
1. Front air bag sensor and bracket (14B005) -- left-hand side of radiator support (16138).

2. Right front air bag sensor and bracket (14B004) -- right-hand side of radiator support support.

3. Right rear air bag sensor and bracket (14B007) -- right B-pillar.

4. Driver side air bag module (043B13) -- steering wheel (3600).

5. Air bag sliding contact (14A664) -- behind steering wheel.

6. NOTE: A thermal fuse is built into the air bag diagnostic monitor (14B056). If a short to ground should occur in the air bag deployment circuit, the microcomputer in the air bag diagnostic monitor will send a signal to the fuse causing it to open. The open fuse removes all firing power (battery/backup power) from the deployment circuits. This prevents unwanted air bag deployments due to damaged vehicle wiring.

NOTE: Code 51 is displayed whenever the diagnostic monitor thermal fuse is open and no other higher priority faults exist (consult Diagnostic Trouble Code Priority Table). Code 51 is normally seen after service of a Code 13 condition.

NOTE: If a Code 51 exists and a Code 13 condition has not been serviced, this means an intermittent short-to-ground exists in the air bag deployment wiring. The intermittent short-to-ground must be located and serviced before servicing the Code 51 condition.

NOTE: Since the thermal fuse is built into the air bag diagnostic monitor, the monitor must be replaced to service a Code 51 condition. NEVER replace the air bag diagnostic monitor without first determining the cause for the Code 51 condition. If the short-to-ground is not properly serviced, the short may reoccur, once again destroying the thermal fuse in the air bag diagnostic monitor.

Air bag diagnostic monitor -- under instrument panel (04320) left of steering column.

 

Yep. Two flash, half second apart, one second pause, one flash, three second pause, repeat.
This is a single cab 1994, does it have a sensor inside the B pillar?

 

Yes.

LFC 21 on this vehicle indicates excessive resistance (or an open) in the safing sensor's ground circuit.

Disconnect the battery, wait one minute. Disconnect connector from the air bag module. Measure resistance between vehicle wiring harness connector pins 16 and 3. Should be less than 2 ohms. If excessive or open, access safing sensor in the B-pillar and check/reseat its ground screw. Re-seat its connector. Repeat resistance check to see if issue was a corroded connection.

There are a few more checks, that will get you started.

 

 

Has nothing to do with this error.