UlttraGauge is currently out of stock. Will get ASAP:
UltraGauge, OBD II Scan Tool & Information Center

 

The problem was resolved by replacing both sensors (Engine ECT + radiator out) and and the thermostat.

Interpreting the error code was somewhat confusing: "P1118 Radiator outlet temperature thermister high, Thermostat reading above 140 °C (284 °F)"

Since the warning does indicate a problem with thermostat leakage I would expect the engine being too cool rather than too hot, but if the radiator out is above 140C then the engine must be really hot which would not relate to thermostat leakage, but may be to a thermostat that doesn't open sufficiently.

That said, the issue seems to be fixed now and I don't think the engine was overheating, because the was no warning on that. Neither temp gauge, nor another warning light.

 

Thanks for the followup, you probably saved you engine...