Well, it turns out there is nothing wrong with my Autotap AT-1 scan tool after all. I had recently installed the Autotap v3.1 diagnostic software on the laptop I was trying to use with my Autotap. I went back to the B&B Electronics Autotap website, and I noticed that their v2.05 software is listed as being "for v1 hardware". When I had downloaded the newer software, I did not see anything on their website saying that it would only work with newer Autotap hardware, but I guess that's the case. Since I have the original Autotap (circa 1998), and I've used the older software successfully in the past on another laptop, I uninstalled the v3.1 software and downloaded & installed v2.05, and voila !
Once I got the autotap to communicate, I found two history DTCs set that relate to the ECT sensor...
- P1115 (ECT Sensor Circuit Intermittent High Voltage)
- P0118 (ECT Sensor Circuit High Voltage)
Just as I had suspected !
I guess what they mean by "high voltage" is that the PCM was seeing a voltage too close to 5 volts to be considered a valid temperature measurement.