I bought a 2008 Nissan Versa LS last month. It has 235,500 kilometres on it but the engine and transmission run quite well. Last week, the “Service Engine Soon” light came on when I started it up after work. The fuel tank was almost empty so I filled it up and cleaned out the gas cap and o-ring but the warning light is still running.

So I bought a Millennium 50 port scanner and plugged it into the OBDII and got DTC p1421. I did a quick O2 sensor scan with the device and I can see and collect data for one of the O2 sensors but not the other.

Is this a indicator that the sensor that isn’t readable is damaged and the cause of the warning light? To experts and gurus, this might be obvious, but two different websites that appear to be reputable tell me two different things.

One site claimes it’s the air filter, while the other claims it’s a an issue with the ECU among other things. I’m not a mechanic so I don’t know.

Is there an issue with buying a used Nissan-built O2 sensor on eBay?

Thanks in advance.


I tried to run tests on the O2 sensors with the port scanner, Bank 1-Sensor 1 is unable to be read by the scanner. Bank1-Sensor2 appears to be fine. That is making me suspicious.

I checked the air filter and the filter is new. It appears that the dealership replaced it.

My Dad wonders if the warning light is on because of a oil change counter not being reset at the last oil change. He thinks that the engine’s computer might be warning that the engine is overdue for a oil change, even though it was changed a few months ago. Is that a possibility?

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    Welcome to Motor Vehicle Maintenance & Repair! Which O2 sensor is it reading? A P1421 is a cold start issue ... shouldn't be anything to do with the O2 sensor. First thing I'd do is look at the engine air filter ... Commented Apr 6, 2019 at 23:04
  • Thanks for your help. You are correct, it is a cold start issue. I updated my original question. Commented Apr 7, 2019 at 19:52
  • What reading do you get from the fuel pressure shader valve with the car on, and the car running. also, how fast does the needle rise on the gauge, it may be your fuel pump or you may just need a fuel injector cleaner.
    – user38183
    Commented Apr 7, 2019 at 20:36
  • @hello moto I haven’t done any of those tests yet. It makes me wonder though because when I went to start it up before work it sputtered and died twice. I’m going to have to check that. Commented Apr 8, 2019 at 15:46

1 Answer 1


I wouldn't buy a new O2 sensor.

O2 sensors have their own diagnostic trouble codes. Whenever you see an O2 sensor code, that's the time to change the sensor. For example, I had a P0138 code a few years ago, and had the defective sensor changed, which obviously caused the code to go away. Since you don't have such a code yet, don't spend your money on a new O2 sensor.

Note the two O2 sensors are different: one of them is before the catalytic converter, the other one is after it. The sensor before the catalytic converter is used for air/fuel ratio measurement, the sensor after the catalytic converter is used for determining the functionality of the cat. This difference might explain why you aren't seeing data from both sensors.

  • sorry; its been a few months. that "check engine light" persisted for another 2 weeks, before it unceramoniously shut off as i was crusing down the highway. this confirms to my ignorant self that it was probably just the o-ring on the fuel cap. Commented Aug 29, 2019 at 4:18

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