 O2 (pre-cat) sensor failure
I got the Pre-cat O2 sensor failure code in August and I replaced my sensor (along with the post-cat one) and every few hundred miles since then the light keeps coming back on. I went and got the O2 sensor replaced under warranty on Thursday and it worked for the 400 mile drive to Toronto only to have it come on at the border (literally, as the guy was scanning my passport). The sensor is brand new!! Why is this happening? The connections seem good, and it goes on intermittently. Someone help me!!
If it helps, the old sensor (the one I got in Aug) was covered in soot when I removed it. | 
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