My daughter has a 2005 Accord, 4 cylinder engine, with automatic transmission. The car runs good and there is no fuel economy problems. At about 92,000 miles, there was a PO139 O2 sensor error. The dealer replaced the downstream O2 sensor. That fixed the problem.
Several months ago (at 139,000 miles), the MIL came on (error PO139). Took it to her mechanic and they said the downstream O2 sensor was bad, so they replaced it. At 141,000 miles, the MIL light came on again. Took it back to mechanic. They said they had put in a non-CA sensor, so they put in a CA certified sensor. MIL light came on again at 143,000. Took it back to mechanic. They checked everything and said emissions were fine, car was running fine. They kept it overnight and checked everything - everything was good. MIL light came on about 5 days ago, but when off today while my son-in-law was driving to the Honda service center.

The car has been regularly maintained. Today, Honda replaced the air bag inflators and did not find anything else wrong. They confirmed that the ECM had the latest updates.

Here are my assumptions:
The PO139 error will clear on its own once the car has gone through several drive cycles.
Wiring to O2 sensor is OK
If the sensor was being contaminated, then the MIL would come right back as soon as it was reset.
If the catalytic convertor was bad, then the MIL would come on immediately, after the MIL was reset.

Could there be brief moments where the mixture is going lean or rich as a result of bad fuel injector, air leak, etc. The mechanic said that there is nothing else that they can do. We want to sell the car (she got a lower mileage car), but feel that it will be difficult because of the MIL coming on about every two months. Is it possible that two bad sensors have been installed. I am willing to get an Autozone Densor sensor and install it myself to see if that fixes the problem.

I will also be looking at the O2 sensor voltage and comparing it with my readings on my 2004 Accord. I assume that they will be about the same and will be steady, as opposed to bouncing around like the upstream sensor.

Anyone have any ideas as to what could be causing the problem?

 

Ouch. These people are what we in the Industry call, "parts changers" Basically they are know-nothings that replace the part they think is bad with no testing or diagnosing to confirm the fault first.



Absolutely.

In fact the code is defined as: "O2 sensor slow response time", this in of itself, does not condemn the O2 sensor.

You may have an electrical issue causing the code, but the majority of the time this particular code is caused by a small exhaust leak somewhere in the vicinity of the sensor itself.

In order to test the sensor electronically you need the ability the graph the sensors response time. With a breif momnet of the engine running lean, the sensor has to respond with-in .2-.7 seconds otherwise it will code. Keep in mind that the judgement is based of off the Airfuel ratio sensors feedback. This testing is typically difficult to do with out a full functioning scan tool.