H2O Doctor — Without instrumentation more sophisticated than the car's MFD or a ScanGauge, I don't think it's easy to determine exactly when the TCH is in "heretical" mode. In addition to the ICE's speed, one needs to be able to monitor MG1's speed, and also its direction of rotation. I believe that heretical mode is commonly employed when travelling at steady high speeds on relatively flat highways. It should not be confused with "fuel-cut" mode which can occur at speeds above ~64 km/h (~40 miles per hour) when the car is coasting (or, at least, when the accelerator is only very lightly pressed). In fuel-cut mode, all fuel flow to the ICE is stopped, but it is being spun at ~1000 rpm by MG1 (powered by electricity from MG2), in order to prevent it from the over-revving that would occur were the ICE allowed to stop completely. Fuel-cut mode can be detected unambiguously from the fact that ScanGauge shows "open-loop" ICE operation, and the dash's FE gauge simultaneously reads precisely zero fuel usage (0 L/100 km or 60 mpgUS). In both cases, electrical power is flowing from MG2 to MG1. The open-loop indication will clearly distinguish fuel-cut mode from heretical mode.

Stan