Thanks picgifs

Just as I was about to take this in for inspection: the air bag light started to blink and then stayed on steady.  Another light for my dashboard Christmas tree.  The car will pass PA inspection with the air bag light on (weird, but true!), but I think I'd like to be sure the air bags actually work.

When I saw the light blinking, it followed a pattern.  Thinking back to the days when you would count the short/long beeps the BIOS generated during boot-up to diagnose problems, I decided to turn the car on and count the flashes.  I counted four, then a pause, then three, then a longer pause, then the same pattern again before the light stayed on steady.

I'm going to guess the long pause was to mark separation between the message being repeated, this leaves me with the following possible meanings

More Obvious

These seem like easy ways to encode the information, we'll try this first

• 4 and 3
  I'm seeing two error codes, one being four and the other being three.
• 7
  Add the error codes together to get a single number.  Why not just flash that many times?  Maybe it's easier to count two smaller numbers.  Hey, I'm guessing here.
• 43
  Concatenate the blinks, that's the error code.

Obscure

This might be what's in the internal state of the machine for the "obvious" answers above, but I'm not sure if this is where I need start looking

• 2^4 + 2^3 (22) or 16 and 8 or 168
  The numbers are powers of two, get these and do the same bit of addition or concatenation.
• 00001111 00000111: 15 and 7 or 157
  Let's assume little endian for right now, as this yields a small number, I don't think we'd have a "big" number for to use for an error code.

Found an answer...

A little interweb scrounging found that it's the concatenation of the two integers (43) and my front impact sensor on the passenger's side was bad.  But how to confirm?  My standard OBD (On Board Diagnostics) tool didn't return any codes, so I would need something more specific to the vehicle.  Some searching on Google Play revealed an excellent app, FordSys, that will read the magic Ford registers over CANBus (the protocol used to deliver the diagnostic information over the OBD port) and that found the following codes:

• B2296
  Front impact sensor failure, does not say which sensor, but the blink code does.  The common "fix" for this is to remove the wire, check for ground faults, clean and re-attach the sensor.
• B1342
  The ECU has some crash data stored, this is likely the result of the sensor failure.  A replacement is north of $600 or you can have the one in your car cleared for $50 from various vendors. I'm searching for a tool that can do this over the existing CAN network, as this part is buried under the center console. 

Just the beginning

There's still lots to do here, it's unlikely that the sensor itself failed, but that's possible.  Maybe with my mucking under the hood, I've disturbed a wiring harness somewhere.  I can do some testing later to see if I can isolate the problem.  For now, I'll just live with the flashing and try to avoid any head-on or offset collisions.