I'm leaving on a 800 mile trip tomorrow and check engine light came on. Took it to autozone and computer said it was the o2 sensor. Is this something that should be replaced prior to the trip or can it wait?

  • 1
    It will adversely affect your fuel economy. But than again autozone sells parts and does not diagnose cars so something worse could be wrong with it.
    – vini_i
    Jan 2, 2017 at 0:26
  • 4
    Which O2 sensor did it say was a problem? Upstream or downstream? Jan 2, 2017 at 0:36
  • What was the exact code that was set? You should investigate the particular code for your vehicle before you replace any parts. Jan 30, 2017 at 6:14

5 Answers 5


I agree with the others it's hard to start anywhere without knowing the exact code that was set.

Even it was an O2 sensor code that doesn't always mean the O2 sensor is faulty, often there are other issues that affect the O2 sensor performance which cause O2 sensor codes which are often misdiagnosed.

If it was an O2 sensor code relating to range/performance you would then test the sensors under different conditions to see if they are actually faulty and also read the live data in the engine to identify any other potential issues.


You can do either, if you drive in town short distance then leave it, and maybe the long drive will clean it and it will be fine after. It won't cause any problems on the trip except maybe? Your mileage won't be optimum.


Without known the exact code you got, it is impossible for us to gauge if this is an issue or not. For example, if its the heater circuit, then you will probably be OK. If the sensor is measuring rich the whole time, then you will probably not be OK (the catalytic converter could overheat and internally melt/get plugged).


Given that typically cars these days have two O2 sensors, there is some level of redundancy. Also, do note that the car when warming up operates in an open loop mode. The absolutely worst that could happen is that your car continues to operate in an open loop mode even when warmed up. It may mean worse fuel economy and possibly higher emissions.

So, if you ask from Greenpeace, they will probably say that the car should not be driven. But my opinion is that you can perfectly well take the trip and replace the sensor when you have time for that.

  • 1
    There's not any redundancy between the two oxygen sensors because the upstream and downstream sensors measure different things. The upstream sensor measures the oxygen concentration before the catalytic convertor and provides feedback to the ecu to help adjust the air fuel ratio. The downstream oxygen sensor measures after the catalytic convertor and ensures that catalyst is working properly. They are not redundant at all. Jan 30, 2017 at 6:17
  • -1 because like Robert said o2 sensors do not have redundancy and secondly because a bad o2 sensor may cause serious power delivery issues or at least awful gas mileage.
    – Techlord
    May 25, 2019 at 18:22
  • On most control systems designs where pre and post catalyst sensors are used the system can and does, to some degree, use both sensors for fuel control. Both report mixture to the PCM, so it can use both for fuel control if the engineers chose to do so. The only vehicles that I have found that do not use the post cat sensor for some degree of fuel control are late 90's Subaru. Jul 16, 2019 at 6:24

The O2 sensors do a couple of things each. The one above the exhaust is there to tell the CPU, that the air/fuel mixture is perfect and can only work with the ambian temp from the sensor in the water jacket and the Mass air flow sensor in the intake. The MAF tells the CPU how much air is coming thru, the ambiant temp sensor tells the CPU that the engine is warm enough to run in normal mode the crank sensor tells the CPU when to inject fuel for a maximum imput and the camshaft sensor tells the CPU when to set off the spark. Now the O2 sensors tell the CPU that the mixture is perfect and the one after the exhaust checks the amount of co2 and when the ambiant temp sensor tells the CPU it's warm enough then the O2 sensor tells the CPU to open the valve between the positive Crankcase Ventilator and the intake manifold to burn the excess gas. Until it's warm enough then the excess gas goes into a carbon filled tank which holds the gas until it's warm then the O2 sensor tells the CPU to open it up and the car... Runs great . I would not like to have my engine turned into a stationary engine, and that is basically what happens when the CPU doesn't get the signals it needs. It keeps putting the engine into start up mode. Then you get to take it to a dealership to get the CPU reset. $1,700.00 is about what it was when I worked for one. I was just a tow truck driver.

  • -1 You do not understand what the O2 sensor does. It simply indicates if you are running rich or lean. Running rich is dangerous to your catalytic converter. It will overheat. Its not a big deal during startup, but over a long trip (especially at high speed cruise) it could be an issue. As for $1700 for a CPU reset - complete rip-off. 1) resetting the CPU fixes nothing. 2) primary O2 sensor failures are detected right away. BTW, after 2 minutes, whether the car's sensor indicate its warm or not, the car must go out of 'warm-up' mode. That's a federal emissions requirement.
    – Dirk Broer
    Sep 3, 2019 at 20:55
  • That is exactly what I said!
    – CareyD
    Sep 4, 2019 at 21:14
  • I have downvoted this answer due to it not really being accurate. You have made some specific statements there that may apply to some specific cards but in my experience are not typical, and there some statements that are not accurate. The O2 sensor just measures the remaining Oxygen in the exhaust, and tells the ECU to add or remove fuel when it is in closed loop control mode. It only indicates to the ECU how the engine is burning fuel. Nothing else!
    – H. Daun
    Oct 16, 2019 at 20:58

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