4.0L SOHC 2004 4.0L Persistent Check Engine Light

[umpcam]

So all was well until a few months ago, on the way to work, got a Check Engine light. No new vibrations or sputtering or anything with it, just the light. I checked it with an OBD reader and it returned P0128 - engine coolant temp too low to read/register. So I replaced the thermostat.

Some time later, not sure how long it's kind of all running together at this point, the light came back. Checked it again, again P0128. I consulted with YouTube and was made aware that it will throw the same code for a bad coolant sensor. Replaced the coolant sensor and all was well....for about two days then the code came back.

When I clear it, it goes away for awhile and will come back randomly. I'm also starting to notice a pattern of the truck throwing the code when/after I go over a bump, which makes me think it's a wiring issue but I'm no mechanic and have no experience troubleshooting wiring harnesses.

Any ideas or insights are MUCH appreciated!

 

[JJMaine]

Does your scan tool show live data? I'm just wondering if it's possible to watch the coolant temp on your scan tool while you move wires around and see if you can pinpoint the issue? Maybe it's not even possible but just a suggestion. Also I'd check the wires coming off the harness to the temp sensor for any damage.

Did you use an OEM sensor? Not that it's your issue but it wouldn't be the first time a new "parts store" part was bad.

Id also have a look at the pins in the connector and make sure they are pushed all the way into the connector and are straight.

 

[umpcam]

Apologies for the loooooonnnng delay on this update. Since that post I've tried to replace the thermostat again, which resulted in a stripped bolt on the Thermostat Housing, which led to a Thermostat Body replacement, that took me deeper into my engine than any non-mechanic should ever want to go, and finally (hopefully) resulted in a ECT Sensor, Passenger Side, swap that has hopefully *fingers crossed* resolved the issue.

While I was away (lol), I got a Bluetooth OBD reader that I was able to configure to show live ECT data while driving. The temp NEVER got above 154-6 Fahrenheit, though it should be in the 190s in normal driving conditions if the lore be true. Once I replaced the Passenger Side sensor it went up to about 200 F and then, when the thermostat opened, it dropped down to about 160. This sensor is aftermarket, as is the other, but it seems to be functioning as desired.

I'm hoping the issue is resolved, and will update if not. (pray for me!)

Thanks for the assistance JJ!!