My car has a P0115 Coolant Temp Circuit Malfuntion. It's causing the car to think it's never warming up. Sensor data say's the car never really get's about 120 deg. It's goes up and down and sometimes bottoms out at -40 and trips the code. The temp fluctuates quite a bit. Finally took the car to the local Toyota dealer today and sat back in the shop with the tech while he diagnosed everything. The car has high idle and run's very rich. He said it's caused by the car thinking it's not warming up. If you unplug the sensor, the ECM defaults to a warmer temp, around 170 or so, and the car run's like a top. Plug it back in and it runs really rich and will not hardly hot start because it's dumping so much fuel in. He checked the source voltage in the ECM for that circuit which should be 5V, he showed me how it varied from 6.5-8V. If these two wires for the temp sensor are supposed to have a 5V power source it seems something in the ECM is messed up. Does this make sense as to what is happening with the car? Would the higher than spec voltage in the ECM for this circuit cause a abnormally high resistance reading causing the ECM to think the car is too cold all the time? I'm planning to remove the ECM and send it to AutoECU and have them look at it as the dealer said they have sent ECM's out before for injector firing issues and it came back and worked as it's supposed to. I don't want to spend 2k for a dealer ECM.

I'm thinking about warming the car up to opertaing temp and assuming it's around 180 degrees and check the resistance going into the ECM and see if it's what it's supposed to be at 180 deg. If it is I really can't blame anything except the ECM.

-Kelly

 

I had a similar issue (our 93 sc400 drove me nuts trying to diagnose the problem till dealer finally had a look and it turned out to be ECM)., picked up a new ECM from a member here for $40 bucs and worked just fine. These ecm's are readily available, cant hurt to try it first.