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I have a 2000 Honda Civic VP with 187,000 miles. I just changed the upstream oxygen sensor about a year ago and it was running smootly until yesterday when it started sputtering badly and the check engine light came on.
I brought it to AutoZone for a code check and one of the codes was PO135, which I think is one of the oxygen sensors. How do I know which one it is (upstream or downstream). Since I changed the upstream sensor less than a year ago, I think it could be the other one.
Any help would be appreciated.

 
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P0135 is for the heater circuit of the front O2 sensor.
That's the upstream one.

 
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Thanks.

 
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check your exhaust manifold for cracks. mine was doing that and threw that code and low and behold i had a cracked exhaust manifold.

 
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you could also gave a bad connection between the sensor and the harness I has a loose wire and I stuck it back in and it stopped all sputtering.