Automotive Engineer
#1
Automotive Engineer
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 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).
#2
#3
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.
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.
#4
#5
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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").
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").
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