P0128 code

[DLinMI]

Temp gauge is running at 1/4, any way to tell if it's the thermostat or temp sensor? I'd be inclined to believe stat if the gauge wasn't reading 1/4 but I'm skeptical. Just bought it 2 weeks ago, first 4.0, moved up from 99 3.0.

 

[RonD]

Welcome to TRS


Not sure what year they changed the Ranger 4.0l SOHC to a single temp sensor, but it was in the mid-2000's.
Previously there was an ECT(engine coolant temp) sensor, for computer only
And then an ECT sender used by the dashboard gauge only.

When Ford switched to full electronic dashboards, the computers ECT sensor signal was sent from computer to dash board temp gauge, so they only needed the one sensor.

I would first have a look if you have just the 1 sensor, or if you have both a sensor and sender.

Look here: http://bengrosser.com/howto/fordthermostat/
Bookmark this page for future reference
Click on Step 5 to jump to picture of the 2 temp devices side by side on t-stat housing.

If you have both then I would stop by an auto parts store that will hook up an OBD2 reader to your vehicle for free, engine should be fully warmed up which should show about 190degF or 88degC, on the OBD2 reader when ECT sensor is selected, thermostat is rated at 192degF.
If sensor is reading correctly then get a new SENDER, make sure it is a sender NOT a sensor, parts guys get these confused all the time.
If sensor shows under 170degF or 77degC then get a new thermostat.
But computer should turn on the CEL(check engine light) if engine isn't warming up enough.

When you turn on the key does the CEL come on, it should, and then go off after engine starts.
If CEL doesn't come on it may have been disabled, which is illegal in most states , but people selling used vehicles often do that, put in a burned out bulb and then claim "it was working" when sold.

If CEL isn't working have a pin and paper handy and write down ALL the codes listed using the OBD2 reader, do NOT buy anything at this time, parts rarely fix most codes no matter what the parts guy says, lol.


If you just see the 1 ECT sensor then it could be the sensor or thermostat, because of the disassembly required to get to either you may just want to change both.

 

[enjr44]

Temp gauge is running at 1/4, any way to tell if it's the thermostat or temp sensor? I'd be inclined to believe stat if the gauge wasn't reading 1/4 but I'm skeptical. Just bought it 2 weeks ago, first 4.0, moved up from 99 3.0.

Well, according to Ford techs and a couple of members here, the 4.0 SOHC engine eats thermostats. They tend to stick open. And at the same time they say that when you replace it you should only use a motorcraft unit. Which cost an arm!!

All that being said, the way I would test it is to find a long hill, sort of steep. Drive up it at a steady speed (say 40/45). If the tstat is stuck open the temp gauge will rise. When you get to the top, turn around and coast (in gear) back down that hill. The temp gauge will go back down. That would tell me the gauge is working. If it doesn't move I would suspect the gauge sending unit.

On my 08 the gauge was reading low like yours and I did the above test and it went up and back down. And would sometimes sort of wonder around when driving on the highway (depending on traffic). After replacing the tstat the gauge goes to just shy of center and stays there.

And, I did the test again and it needle never moved. However, your mileage may vary!!

 

[adsm08]

Thermostat is a common failure on the 4.0 SOHC. I have seen probably 100 of these codes on that engine and all of them have been the T-stat.