I recently replaced the thermostat and the upstream O2 sen
sor on my 2002 S60. I understand a faulty O2 sensor can cause a catalytic converter to burn out and need replacement.
Well, the Check engine light came back on after the sensor replacement and reset.
What next? Appreciate any help.

 

What's the fault code?

 

I haven't checked the latest. Autozone and Advanced and a garage all agreed that the code before I changed the O2 sensor was for the O2 upstream sensor The latest(continuing) check engine came up immediately after I changed the sensor and came up after the reset.

 

Error codes read(from Autozone)

P0037 HO2OS indicates that the bank 1, sensor 2 HO2S heater control circuit signal had low voltage for a predetermined period of time

 

That is for the rear sensor. You replaced the front sensor, correct?

 

That's right. Guess I should have realized I needed to replace both. Thanks for your response. One more thing: Is there the same urgency to replace it as the front sensor?

 

The preheating element in the sensor is not working. So the sensor may still be reading fine, once it gets warmed up. It'll take longer though since the heater doesn't work.