how ca i figure that o2 sensors must be changed

[user10]

how ca i figure that o2 sensors before the cat must be changed since i do not have a scanner and CEL is not on

my catback is ruined as well as the cat probably and i'm wondering if that can affect my mpg i have 9 - 10 mpg

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excuse my english

 

[dogfriend]

If your catalytic converter is plugged or if your muffler is plugged it could affect your mpg. A mechanic can check this by testing the backpressure with a gauge. The gauge would be attached to the O2 sensor port for the test.

It is possible to test the voltage output of the O2 sensor, but it must be done with the engine warm because it only generates voltage above a certain temperature. If the O2 sensor is not working at all, it should cause the CEL to come on.