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ECU passes emission test with bad O2 reading?

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  • ECU passes emission test with bad O2 reading?

    OK, my first post -- I apologize in advance for its length, but the reason I joined this forum is a bit complicated. I am trying to understand the behavior of the ECU in my '96 Tacoma which was happy until I had a muffler replaced. Immediately (actually a day later) the MIL light came on. I have an OBDkey which showed me a P0420 DTC, and the live sensor scan for the downstream O2 sensor was pegged (stuck at 1.275 volts). I took it back to the muffler shop and they replaced the O2 sensor, but a few days later the MIL light was on again, this time with an additional DTC of P0141 - O2 sensor heater malfunction. I measured the heater and found it to be outside the range specified by Toyota, so I took my ohmmeter to the parts store and found one that was in range. On replacement, the MIL light came on AGAIN! The DTC was again P0420, and the O2 sensor was still pegged. Using a multimeter and opening up the ECU I was able to determine that the O2 sensor voltage was behaving normally outside the ECU, but wasn't getting through the connector into the ECU. And nothing I could do seemed to help. Since I was due for a State inspection, I decided to invoke the North Carolina waiver procedure which kicks in after you've failed an emission test, paid over $200 to have the failure fixed, and failed again. Of course with a P0420 code, the shop replaced the catalytic converter (for over $200). I was sure it would fail again because the O2 sensor voltage wasn't getting into the ECU. But after two days of driving, the MIL light stayed off, even though the OBDkey showed me the O2 sensor signal was still pegged. And to top this off, it passed the inspection with the ready codes set and no DTCs, even though the O2 sensor signal remained pegged. Now I am delighted that it passed inspection, but am totally befuddled about how the ECU could pass with no DTCs while telling me the O2 signal was in a fault condition. Is it possible that the OBD scanner the shop used could reset the DTCs more completely than my OBDkey can? WHAT is going on??? Please you Gurus out there, can you explain it to me?
    Sincerely, Leandra, the lady engineer and geek