John,
As others have suggested, start with replacing the female spade connectors at the sensor end. I’ve had to replace several of these on Bristell NG5s. I like the spring loaded connectors that Sean posted, but a standard new connector will tell you if that’s the problem.
Bristell often labels the CT sensors as CHT sensors. It’s just a digital label problem, and you can re-label it in the configuration menu of the PFD if you like.
EGT can, in some cases, rise as you unload the engine, such as when lower the nose to descend. It sounds backwards I know, but EGT represents heat energy wasted out the exhaust. CHT is the opposite, and represents energy being used for thrust. In other words, a heavier loaded engine will typically see higher CHT and lower EGT, and vise versa. I only bring this up to say you should focus on troubleshooting the CHT sensor first, without being distracted by the EGTs.
At the Garmin end, you will find the CT and EGT probe wires terminated at the Garmin Engine Information Module (Garmin GEA-24). From that module, information is sent to the Garmin glass via the Garmin Controller Area Network (CAN). Specifically, the wires will be terminated on the 25 pin Sub-D connector called out by Garmin as connector J242. You can download the G3X Touch installation manual and check out section 26.5.2. for the connector pin-outs. However, these connections at the Sub-D connector are crimped pins and are typically quite reliable.
In one instance I’ve seen a failing Garmin GEA-24 cause erratic readings, but that’s very uncommon.
