Prior to a CA smog test, I replaced both of my O2 sensors (I had a P 0137 code). The test passed fine, and my miles per gallon improved, until about a month ago. My economy went down roughly 12%. I've been too busy to investigate why, and not driving that much anyway. Yesterday, I got a check engine light, with codes P 0137 and P 0136 2, both O2 sensor codes.

Of course it could be the wiring, which I'll check. But assuming it's not, my question for the forum is about the source I selected for the parts: eBay. Napa and some more reputable stores had the O2 sensors for 3 or more times the price of the parts I bought on eBay. They looked ok, no signs of rebuild, and the resistance on them was fine. All that said, is there a quality problem / high failure rate with cheap O2 sensors? I did the same thing years ago and they lasted years.

Thanks for reading.