I have 1997 Supercab F-250 with 460. This fall I started to have problems with truck not starting until three tries. Then it started running rough around town. I brought it home and pulled the code. " 1-2-6 " Book said Mass Air Flow Sensor. This was the second time in three or four years since owning truck. This is the sensor by the heater on firewall not one that fits in air flow venting. I don't have a sensor in my air venting.
I also cleaned distributor cap and checked plug wires. I replaced all of these when I first got truck. For about a month truck was fine then when I would start it, it would start and Check engine light will go off and then it flashes three times and the truck revs up until it is put in gear.
I can't find anything on this problem about flashing code with out using wire to short OB I and count flashes.
Any ideas. Do I need to re-check the codes and count the flashes? I'm worried that this beast is using more than it's 8 mpg because it is metering more gas than it should. It's the middle of winter and not a good time for me to stand on ladder in the cold while fiddling with things that may not need fiddling.
This truck has two gas tanks and I need to replace the rubber part from filler to rear tank as it is leaking gas when tank is full. I thought I would wait for spring and just not use rear tank till spring. If that is the problem, I'll weasel my way into my brother-in-laws heated garage to fix it.
Right now, I am using truck for in town running but would like to run a couple hundred mile trips soon.

 

I have never seen an OBD-I vehicle flash out codes without grounding the STI pin on the data link connector. If indeed it is somehow doing this, perhaps that lead is grounded somehow, Code 126 correct definition is: MAP sensor out of self test range. The MAP sensor is located where you stated. MAP is an acronym for Manifold Absolute Pressure (MAP) sensor. It is a critical sensor for the computer to properly run the engine. It's output is a variable frequency, not voltage as manual manuals state.

For reference: Manifold Absolute Pressure Sensor (MAP)

I would run the Key On Engine Off (KOEO) tests and Stored Code display to see if anything else shows up. Verify the STI lead is not grounded.

 

The only time(s) I've eve seen an OBD-1 vehicle flash the fault codes at start up was when I'd forgotten to remove the jumper....

 

I'm having a hard time posting. It keeps telling me I've timed out. Any way I first tried to post that in cold and wet I went to start my truck and it had dead battery. After days of cold and wet, I went out this morning temp is up to 15 degrees.

Guess what? I left the grounding wire on. (A real feel stupid moment.)

I have battery charger on it. I think the radio drained the battery so I have to check that out.

Thanks for pointing me to errors of my ways.

 

You aren't the first one to do that.... Been there myself on my old SHO.

 

Glad you found the self inflicted cause of your code flashout issue!