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I got a P1273 code?

I got an 04 Nissan Sentra and I got a (P1273 Nissan - Air Fuel Ratio Sensor 1 Lean Shift Monitoring Bank 1) code that popped up on it. Looking to see what the probable cause for this code to pop up? Is this a rare code that popped up on this car? There’s no noticeable symptoms going on with the car.

I would have the code reset if it’s not blinking. That’s what I do. If it comes back…take your Sentra to a good independent mechanic and have it checked out. There are so many possibilities that it makes it impossible to diagnose the problem correctly unless you have experience in this area. Guessing will just cost you money and “shotgun maintenance” is never a good idea.

  • Air fuel ratio (A/F) sensor 1 Bank 1
  • Air fuel ratio (A/F) sensor 1 heater Bank 1
  • Fuel pressure
  • Fuel injector
  • Intake air leaks
  • Air Flow Meter (aka MAF sensor)
  • Bad Catalytic converter…

I did clear the code and it came back 7 miles later. Also forgot to mention the car has a small exhaust leak on the flex pipe under the car, But was told by 1 person the code is not on because of that.
Would I be able to test the possible problems with a Obd2 scan tool with live data or some other way? Because I have read it could be 02 sensor, MAF sensor and have heard that you can get reads for that off an obd2 scan tool with live data?

Get that exhaust leak fixed first and reset the code. Small things…even an exhaust leak or dirty air filter can cause the CEL to come on. Step by step here. The exhaust leak can be dangerous in the right conditions.

Second that exhaust leak repair. How can an oxygen sensor function properly when there’s a leak? The answer is it can’t.


“the car has a small exhaust leak on the flex pipe under the car, But was told by 1 person the code is not on because of that.”

Don’t take automotive repair advice from that guy anymore

they apparently don’t understand how oxygen sensors work. And they apparently also don’t understand the concept of false air and fuel trim

As others have already stated, the upstream oxygen sensor is picking up that exhaust leak, and is seeing excess oxygen. It interprets this as a lean conditon, and is trying to correct for it. Because the fuel trim correction is maxed out, you have that fault code

I’ve worked on several vehicles, where an exhaust leak upstream of the upstream sensor caused lean fuel trim codes

And I’ve also worked on vehicles, where those fumes were sucked into the cabin. To the point that the driver would get drowsy, because the exhaust was displacing oxygen in the vehicle’s interior. A decidedly unhealthy situation to be in, I might say

Ok, so I planned on writing and stating that this problem was fix because I had a p1273 code and a pending p0113 and was told that the pending code could be related and to worry about that 1st. So I did because that seems like it would be the easier and cheaper fix. So I looking up what to fix on a p0113 code and 1 thing was air filter. I knew b4 this engine light that the air filter was dirty but not that bad looking.
So I change the air filter and erased the code. (If I erase this code and drive it the light comes back on 7 miles exact.) Drove it 100 miles and had my live data scan tool hooked up to it and noticed the monitors were reset and no light. But got to 500 miles or so driving since erasing the codes and the light came back on.
Now the p0113 code is gone but I still have the p1273 code and also a pending p1273 I think. (Not sure if that pending code is a big deal because it’s the same as the hard code. I did have a p0335 code in the past and seems like the same hard and pending both come on when the light comes on fresh from it being erased. Drove it couple times on that p0335 code and all I had was the hard code, no pending, And when I put the sensor in at the time, no more code, fixed.
I do have a small exhaust leak in the flex pipe, and an issue with the car starting up when it’s been ran and sat around for 30 mins or more, but if I turn the key 2 times to prime the pump it starts a whole lot better, cold starts not needed to turn key, not bad at all. (But seems like now it’s gotten worse, need like
3 or 4 key turns to start normally.
So I was able to drive it for a bit without the light coming vs now when I erase the light an exact 7 miles the light comes back on. Curious what you guys think what the problem could be now from what I stating. Am I able to at least rule out the 02 sensor being a problem since I was able to drive it 500 miles. I haven’t tackled the exhaust leak because it seems like it’s going to be a pain and expansive if that is the issue with the code. Curious from what I stated if it helped narrow things down a bit.

How are you supposed to help someone who won’t fix an exhaust leak ?

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I told you six months ago . . . ! . . . that an exhaust leak upstream of the upstream sensor will affect its operation

“Am I able to at least rule out the O2 sensor as being a problem”

see above

I would say your P1273 code may very well be caused by that exhaust leak

I also told you about the dangerous possibility of fumes . . . from your exhaust leak . . . being pulled into the cabin, making you drowsy

fix that exhaust leak

Otherwise it might turn into a P0420, something OP would be better of without.

Update: I just want to give the results on this problem. I did get the flex pipe replaced (at a shop) months ago and put over 7,000 miles after the replacement of the flex pipe. The leak in the flex pipe was the problem because the engine light is gone.

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Thanks for letting us know. Your hard to start and having to keep turning the key multiple times is a bad fuel pump.

Thanks for the update OP. Good for a reminder to us all. Modern electronic fuel injected cars just don’t run properly if there are any air leaks.

I could be a real smart alec and say what I feel needs to be said

But I won’t

Read our past comments . . . especially mine, because I said it several times . . . and you can figure out what it is I would like to say at this time, but won’t


Hi. So I have a 2004 Nissan Altima and need to pass emission on the car but the code p1273 show up right after I replaced the cat and the flex pipe. So I replace both sensors and still light on… I don’t know know what to do cat and flex pipe are new… help please…