Method to detect the presence of a liquid-cooled engine supplemental heater

A method for detecting an engine block heater includes block heater detection logic and a data table containing calibration thresholds. The method determines a time-series of delta (Δ) engine coolant temperatures (ECT), which represent the respective differences between a start-up ECT and a plurality of operating ECT values measured thereafter. The method integrates the time-series to obtain a Δ ECT integral. The method also maintains an integrated engine speed parameter accumulated over the course of the detection period. Throughout the detection period, the method retrieves a threshold value from the table based on the integrated engine speed (Y-axis) versus an elapsed time (X-axis) since the detection period began. When the calculated Δ ECT integral value exceeds the retrieved threshold, the method determines that the block heater was operated, which can thereafter be used to suppress sensor rationality testing or reporting of results thereof.