On our '94 Bonneville SLE (3800 engine) after some engine warm-up, the check gauges light illuminates for approx. 5 to 10 seconds and I hear a chime, but no issue with the temp, oil, or voltage gauges. Also, the electric engine cooling fan(s) are "on" when the check gauges light is illuminated.

Using an OBD Diagnostics cable connected to the vehicle's 16-pin ALDL connector and my laptop, I monitored the engine coolant temp sensor. TunerPro RT shows the sensor temp increasing from a steady 154F to 296F in just 0.2 seconds. The temp was then erratic mostly above 250F for 5 to 10 seconds before dropping back down again in 0.2 seconds to the steady 154F. The engine cooling fan(s) were also "ON" in the data log when the temp was high.

I'm trying to determine if the sensor or ECM needs to be replaced. I'd rather not remove the difficult to get to sensor and test it if I can avoid it. I also know that this vehicle is supposedly notorious for ECM failures. I replaced the ECM 12-years ago (32K miles). Current mileage is only 81K.

Any help would be greatly appreciated.