CT Emissions first failed my 97 E420 w/ 187K miles with error codes:

PO455 Evaporative Emissions Systems Leak Detected (large leak) and
PO442 Evaporative Emissions System leak Detected (small leak).

I changed the gas cap and reset the codes and ran it for a few days and the CE light came on again so i took it to Autozone to read and they said it gave:

PO150 circuit condition (heated oxygen sensor bank 2 sensor 1)

Explanation: "The powertrain control module uses the oxygen sensor to calculate the air/fuel ratio in the exhaust.

Probable cause:

1 oxygen sensor defective
2. fuel system running very rich or lean check other codes first
3. engine misfire condition- repair
4. fuel pressure very high or low-fuel pump or pressure regulator.

How is it that this new code appears and wasn't in the first reading? Is the oxygen sensor easy to replace DIY? Thanks.

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