I just installed 2 x 18mm long, tapered spark plug defoulers. I disconnected the neg battery terminal, left it off for a minute and put it back on. Started the car up and immediately got a P0141 code.

Is the P0141 mean the sensor is bad or that it's getting an abnormal reading?
Shouldn't the CEL for the test pipe come on after 3 drive cycles?

 

Complete heater malfunction P0141 means the sensor is not working at all. Make sure you connected it right and pins/contacts are ok. Also, grab the sensor while holding the washer and check if it rattles when you shake it. You may have broken the ceramic element inside it which would throw this code.

If you want to get more detailed, warm up the car (fans cycle) and then shut it off. Measure if you have resistance between the bottom two pins of the sensor (looking at it with the tab upwards). Should be about 5 ohms. If you get nothing, it's broken.

 

Okay that makes sense. Long story short, the O2 sensor fell out and was dragging on the ground at highway speeds.
Thank you sir.