Cat with "low temperature reading"

My 2001 Nissan Frontier has had a check engine light on for about 8 years with a 420 code, but has always run well and gotten ~28-30mpg (it’s a little V4, manual). I finally took it in to get it fixed and they told me that the catalytic converter was bad and that the O2 sensors also needed to be replaced. The mechanic said something about the sensors reading about 100 degrees too low…

Can someone explain what this ‘low temperature’ comment might mean? And what would cause this to happen? I am simply curious and couldn’t find an answer on the google. Thanks!

(If it means anything, I bought the truck used with 30,000 miles on it. I had a mechanic look at it when I bought it and he said it looked great. The current mechanic just told me that the catalytic converter had been replaced at some point. Could someone have put something on incorrectly prior to my ownership? Again, just curious because I’d like to understand my car a bit better!)

Have you seen this?:

If the O2 sensors are bad, how do they know the cat is bad too? Properly functioning sensors are the only way to accurately measure the cat’s performance.

Anyway, insightful’s link posted above is probably the right answer.

If you’ve had a Check Engine light on constantly for about eight years, it’s kinda hard to tell what else might have gone wrong that may have turned the Check Engine light on in that time.


O2 sensors don’t measure temperature, they measure O2 concentration in the exhaust gas. But they need to be hot enough, otherwise they don’t provide an accurate measurement. W/some cars they get their heat from the exhaust gas. W/ other designs, the O2 sensor has an electric heater built into it. I expect your 2001 Nissan has some of the type with electric heaters, and the mechanic is saying the electric heater isn’t working for some reason. So the O2 sensor isn’t hot enough to provide an accurate O2 reading. Until that is fixed, there’s no way of knowing if you have a cat problem or not. Typically the no-heat to the cat O2 sensor problem is either the O2 sensor itself, or the heater circuit. It could just be a blown fuse for the circuit that supplies the O2 sensor electric heater. Or the connector is corroded.

Admittedly, I’m now a little nervous that the catalytic converter was replaced prematurely. I sent the mechanic that bulletin (thanks for posting it, hopefully it will help the next person who brings their car/truck in). Thanks for everyone’s insight - I really appreciate it!