If the pre-cat O2 sensor fails, the ECM is no longer able to determine whether the mixture is rich or lean. A pre-cat O2 sensor cannot fail silently, the CEL (check-engine light) will switch on. I am wondering whether the ECM has any other way for continuing to even try to hit the stoichiometric point, or will the engine be just running on either a rich or a lean mixture?

If the mixture remains always lean, the engine will overheat, then it's a matter of where one is: in cold weather the cooling system may be able to catch up; in hot weather it may not be able to keep up, and the cat may be damaged. Or the mixture will become rich and the driver may notice reduced mileage.

  • Rich is less likely to overheat than lean. The extra fuel helps remove heat from the cylinder.
    – rpmerf
    May 19, 2016 at 12:24
  • I think you mixed the conditions up, though. A rich condition won't make the engine overheat (extra fuel has a cooling effect), but will decrease mileage and can damage the cat in the long term. A lean condition shouldn't reduce mileage, but will may induce knock because of the increased temperatures. Either way, if the O2 sensor fails and the ECU recognizes that, the ECU should keep the engine in open loop mode - there is no way to maintain a ratio. May 19, 2016 at 12:24
  • Fixed the, um.., misunderstanding of the lack of understanding.
    – Calaf
    May 19, 2016 at 12:50
  • 1
    If the O2 fails the ecm defaults to other sensors (MAP, MAF) to keep the fuel mix close, and it will by default try to keep it little rich. This is a very broad question as every ecm for make model and year has different software and strategies to deal with a bad O2.
    – Moab
    May 19, 2016 at 15:09
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    @Calaf I like this first part of this question. It brings out an often misunderstood principal of modern fuel control algorithms. May 20, 2016 at 3:00

2 Answers 2


Both pre and post catalyst "Oxygen" Sensors report exhaust fuel mixture to the PCM. So the PCM can and does use both to sense and adjust mixture. The rear sensor gives a better average mixture because the catalyst is a mixing chamber. In newer designs the rear sensor is used more for mixture sensing than the pre-cat sensor. In these designs a pre-cat Air Fuel Ratio sensor is used to sense individual cylinder mixture and misfire.


I think it very much depends on the ECU you are using. On early Volkswagen systems, they feature a pre and post cat lambda probe in the US as this is mandated but only set mixture based on the pre cat probe. Makes sense I guess as the post cat probe won't tell you much if the cat is working correctly.

In the UK we don't have check engine lights so a probe can fail silently but I can tell you from personal experience that the failed state is for the engine to run rich. Fuel consumption drops considerably and emissions typically rocket to ten times over the prescribes limit by UK vehicle inspection standards.

Remember, a rich mixture is considered a safe mixture for engine components when compared to a lean mixture.

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