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Replacing 02 sensors frequently

steve mezzle

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Hi Guys

Can anyone make any suggestions with this issue please on my 1998 Toyota avensis

I had my engine management light come on last week. Fault code 21 (oxygen sensor). Replaced with a second hand sensor. light went off  for 1 day and then management light come on again with same fault code.

The sensor that was replaced was the pre cat sensor (under the bonnet)

The car runs OK, no noticeable power loss , fuel efficiency seems the same.

replaced the spark plugs at the same time as the sensor as they were due anyway

New cat converter was fitted in July.

I tried to follow an online test i found to check the sensor heater and the heater appeared to be open circuit ( high ohm reading), havent tested this new one yet

Checked the voltage to the sensor was 12V and has ground


Ive done some internet searching and have found the following possible causes , but as i dont know enough about cars perhaps someone can narrow down some of these possible issues based on the info above.

Failed fuse, Exhaust leaks, running rich, damaged wiring back to ecu, faulty ecu, fuel injector, fuel pressure regulator, faulty thermostat, faulty mass air flow sensor, air leak exhaust manifold


Thanks in advance


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