I have a 2003 F 150 Heritage Edition with the 5.4 L engine.
I was driving up to Wisconsin from Charleston and the service engine light came on and has stayed on.
The codes I am getting are P0125 and P0420.
The P0125 code is not making any sense to me as the engine was at normal operating temperature at the time it came on (I was cruising on the highway at 70 mph for at least the previous 250 miles). The engine still warms up normally with the gauge reading just under the halfway point and I have plenty of heat in the cab (In fact I have to turn down the temperature).

 

I don't remember whether your engine has separate sensors for the gauge and the PCM. If it does, the one for the PCM isn't working.

The P0420 is low catalyst efficiency passenger side. You may be starting to plug up the cats.

 

You need to use a Scanner to look at the engine temperature to see what you really have.
if the temp sensor resistance is way out of range, it will cause a code to set. Then you find out why!
Use the Scanner to look at the (rear) Ox sensor passenger side for action. If it tis varying in voltage signal over a wide range, the cats ahead of it are not storing Oxygen and are at the end of their life. That's all it does.

Otherwise it's all just guessing without investigation.

 

The P0420 might be related to the P0125. Your truck might not be entering closed loop because of the bad coolant sensor thus messing with the PCM.

I'd get the P0125 fixed first and clear the DTCs and wait to see if the P0420 returns.

If it returns, you got more fixing.

 

P0125 codes are usually caused by a thermostat that is partially stuck open. Based on the testing criteria for that function, it's HIGHLY unlikely that you'd see that code during steady-state driving, it's supposed to only be set during the engine warmup-up cycle and isnt't even checked after about 13.4 minutes after engine start-up. FWIW, the 2003 5.4L uses only the CHT sensor, ECT data is inferred from the CHT. The gauge on the dash is going to read mid-range any time the PCM determines the CHT is within operating parameters. You'd have to put the cluster into DTM (or use a scan tool) to see what the actual reported (inferred) engine temperature is.

A P0420 does not indicate that " You may be starting to plug up the cats ". That code indicates that the PCM thinks the cat is no longer functioning as intended. Huge difference. Most likely, if the OP was indeed cruising at steady state outside the limits of the thermostat monitor's range, this would have been the primary reason for the MIL coming on.

BTW, there's no such thing as a "2003 Heritage" F150. The "Heritage" models were 2004 models that were carryovers from the 2003 model year. It had to do with the process of model change-over one production plant at a time. One plant converted before the other. The first was pumping out the '04 NBS versions while the other continued pumping out leftover '03 design trucks that were sold as 2004 models (OBS, aka "Heritage").

 

https://www.f150online.com/forums/ot...-produced.html

 

If I remember correctly, Ford was worried that the new 2004 style wouldn't be that popular so they continued making the 03 models for a while.

 

the 04 heritage was the way ford was able to get rid of new old stock 2003 body styles.

 

I keep forgetting about that "limited edition"

 

Don't worry... I do the same thing. Folks like you and glc correct me...

 

Has it ever happened before or since?

 

The 04 NBS trucks were built in the Kansas City and Norfolk plants. The 04 Heritage trucks were made in the Oakville Ontario (Canada) and Cuautitlan (Mexico) plants.

Anyway, back to the original topic...

 

slightly, yes.....

ever seen a light duty f250? It was an f250 with an f150 body

manufacturers do this, isn't a new thing. Isn't just ford either.

 

The light duty F250 was actually a real F150 with a 10.25" rear and 7 lug wheels. It became the F150 7700 package. It had nothing in common with the F250.