PO128 Thermostat code help

Hello, I'm new to the StangNet forum, but I've been doing DoorDash for a two months or so, and my mustang started overheating. I first suspected the thermostat, so I changed that and added new coolant. But whenever I was at a stop or basically not driving down the highway the engine would slowly overheat. So I thought, well it can't be the thermostat because it's new now, so I went to my Chilton Mustang Owners manual and suspected that it might be the engine coolant temperature sensor. Finding and removing the engine coolant temperature sensor was a saga of its own. Mine was positioned between the firewall and the back of the engine. So I changed that out. It screwed in a little tight which I thought was funny, but I was pretty sure it was the right sensor for my model. I take the stang out for a test drive, but bizarrely enough its not overheating anymore. It's now under-heating. I could tell because when I turned the air conditioning to hot, cold air was coming out. I thought to myself, well I had cleared the codes in the engines computer using my obd, so maybe it needs to collect more data and it'll return to normal. So eventually after a few drives, it gets to a lukewarm air temperature. Except now I'm getting terrible fuel economy. I think when I drove my car for like 15 minutes it dropped a quarter of a tank, but then started to slow down. I didn't notice any black smoke from my exhaust and I went back to the Chilton manual and to my obd. The obd threw me a Thermostat code, and again I just changed the thermostat. So it has to be something with the ECT sensor. Maybe I screwed it in too loose or too tight or something, but why that would make my engine undercool, I don't know. Any suggestions about what might be the problem.

Noobz347 said:

You have two different issues going on:

1st issue: Car overheating while sitting in traffic. You changed the thermostat but no change. Later, you changed what sounds like, the [coolant temperature sensor] which fixed the problem. It makes total sense since it is responsible for sending the signal for the electric fans to turn on and at what rate.

2nd issue:


This is a [blend door] issue and not related to your first issue. You either have a vacuum line beneath the dash or up against the firewall, under the hood, that is either torn or disconnected. The possibility also exists that your [blend door actuator] is either disconnected or malfunctioning. Neither of these conditions would throw a code unless the vacuum leak from the line being disconnected, is so severe that it causes an [engine lean] code.


Black smoke is another issue but that could also have been the result of other tinkering.

Click to expand...

I didn't notice any black smoke, but when I was trying to access the engine coolant sensor near the firewall, it's possible I could have torn something. The access was so tight that it was hard to avoid knocking anything with my ratchet. Perhaps I should have disconnected more of the harness to avoid that. But I'm still trying to understand why the engine itself is underheating. Could that also be because of the vacuum line disconnect