On warm-up there is a big temperature difference between my aftermarket gauge and the engine coolant temp sensor. When it gets up to operating temperature the difference is much smaller, but still there. FWIW, I have Omoni branded gauges, not Defi. I'd definitely trust the OEM sensor over any aftermarket ones, the ECU uses a 32 entry calibration table specifically for that thermistor type. How accurately are aftermarket gauges calibrated?