My daughter has a 1998 Toyota Rav 4 with 109,000 miles on it. I generally see her and the car once a week. About a year ago, the check engine light came on, but there was no DTC. It went off after a few days. It would come on for a few days or weeks and then stay off for a few weeks or months. Sometimes the idle was abnormally slow. About a month ago, right when it was due for inspection, the check engine light came on and it had a DTC; P0171, fuel trim system lean. I did some research found that this is usually caused by a dirty or bad mass air flow (MAF) sensor. I was all set to clean the MAF but I found it didn't have one. My daughter said the gas mileage was much worse. I decided to start replacing stuff. The next week it had a new DTC; P0136, heated O2 sensor circuit malfunction bank 1, sensor 2. I could not find an oxygen sensor, but I replaced the spark plugs and air filter and added fuel injector cleaner, as planned, and cleared the DTCs. She said the fuel economy was back to normal, so I thought maybe we don't need to replace the O2 sensor. A week later the check engine light came on again. There was no DTC. I swapped cars with my daughter and ordered a Bosch "OEM" oxygen sensor from Auto Zone. Two days later I had the sensor screwed in, but I could not plug it in. Although at first glance the old and new plugs looked the same, they were keyed differently. I returned it. Advance Auto did much better; they got me the wrong part in one day. I am very disappointed that car parts stores and web sites will have a wide variety of incompatible parts, flanged and unflanged, one wire, two wire and four wire, and they say they will work on this car! Today the check engine light went out by itself and it passed inspection.
What should I do? Should I change the oxygen sensor? Are all screw in, four wire sensors the same except for the connector?