Q: My car shows a P0420 code and the engine coolant temperature reading (via torque pro app and elm327 Bluetooth reader) stayed at 84.2 degrees

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How low can the ect read, I'm pretty sure a faulty ect is my issue that or an o2 sensor, because the cataylist should still be fine with the mileage on this car, but I have not ruled it out completely, car was parked for roughly 5 hrs before checking then I first done KOE to get code, cleared and monitored PIDs with engine running approximately 5 min (I feel the owner of the car has been swindled by a shop) Thanks

My car has 90000 miles.
My car has an automatic transmission.

Hi there - "boundary measured engine temperature," or the lowest temp reading a sensor can transmit, can be different based on each temperature sensor. The issue here is what is the true engine temperature? Is the thermostat getting the engine up to its design temperature (something near 205 degrees Fahrenheit/100 Centigrade). Can you cross-check the engine temperature with a laser thermometer, cockpit temperature gauge (middle of the gauge usually), or other measuring device? If the engine is truly coming up to 200 degrees (F) or so, then yes, the ECT can read "low" to the computer, causing rich mixture running. This could cause a low catalytic converter efficiency problem if the engine has run rich for a while.

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