Ford Flex P0031:  O2 Sensor Heater Control Circuit → Low B1S1

P0031 is a generic code, which means that it has the same meaning for the Ford Flex as it would any other OBDII equipped vehicle. As far as diagnostic trouble codes go, this is a relatively easy one to diagnose. It is usually caused by a bad oxygen sensor, or a wiring issue.

P0031 Ford Flex


P0031 Definition + Sensor Location

Ford Flex P0031 Diagnosis

Bank 1 Sensor 1 Location

  • Bank 1 is the side of the engine with cylinder 1. If you have a four cylinder engine or inline six, it’s the only bank. 
  • Sensor 1 is upstream of the catalytic converter.



When the engine is cold, it has a hard time getting a good reading from the Flex’s oxygen sensors.  Without a good reading, it is difficult for the ECM to determine the proper air/fuel mixture to give the engine.  The heated element in the oxygen sensors helps to get a more accurate reading of the oxygen in the exhaust.

The job of the O2 sensors in your Flex is to measure the level of oxygen in the exhaust so that the ECM can make changes to the air fuel ratio.  As stated above, the oxygen sensor has a heater that helps to give it a more accurate reading.  When this heater is not operating, it can affect the way that the engine runs.  With P0031, the O2 sensor is informing you that the heater circuit resistance is lower than the normal operating range.

This code will often appear with P0171.  It indicates that the Flex has a lean condition.  If these are the only two codes present, fixing what is causing P0031 will more than likely clear the P0171.



P0031 Symptoms:  Ford Flex

P0031 is most often not accompanied by any noticeable symptoms as long as it is the only code that is currently stored in the ECM’s memory.  If there are symptoms, they’ll be:

  • Service Engine Soon Light
  • Diminished Fuel Mileage
  • Rough Running Engine
  • Raw Fuel Smell Coming From Exhaust (particularly when cold)

If the engine is running rough, it very well may be accompanied by the P0300 code.  This indicates a random misfire in the cylinders.


P0031 Causes:  Flex

The most common cause of P0031 is a bad oxygen sensor.  It’s, not the only cause though.  A quick examination of the Flex’s physical wiring harness, as well as the voltage coming from the heated O2 sensor can save you from buying a sensor that you don’t need.


Wiring Issue

Due to the location of the oxygen sensors (under your Flex, on the hot exhaust pipes), the wiring harness around them is easily susceptible to damage.  Hot exhaust can make the wiring harness brittle.  A visual inspection will often uncover exposed wiring (that’s shorted) or wiring that is broken altogether.  Pay particular attention to anywhere that the harness comes close to touching other metal, and where it is closest to the exhaust.

If the harness looks like it is intact, you’ll next want to to test and make sure that the heated oxygen sensor is getting power.  You can take a simple test light and check to see if the harness is getting voltage.  If it’s not, check the fuse.  Make sure to test the ground circuit for continuity.

If the oxygen sensor is getting power to it, you can check to see if it is getting the correct level of voltage/ohms.  Here’s a great video from Ratchets and Wrenches on how to test the wiring on a heated O2 sensor:


Oxygen Sensor

It is highly likely that the oxygen sensor has gone bad when you get this code.  The good news is, they are affordable and easy to replace.



It is technically possible that the ECM has gone bad when everything seems to be working, but you still get this code anyway.  It’s not very likely though.  Here’s more on the symptoms of a bad ECM.


Conclusion:  P0031 Flex

This code is relatively straightforward to diagnose.  If there is anything that you would like to add, please feel free to leave a comment below.  Best of luck to you.  If there is anything you would like to add that could help the next person with their Ford Flex, please leave a comment below.

1 thought on “Ford Flex P0031:  O2 Sensor Heater Control Circuit → Low B1S1”

  1. Well citrus ford here in Ontario CA says were full of it. They say the sensors don’t go bad. Been having issues since around 74000 miles. Have ford extend warranty. An they replaced the O2 sensor. Charged for Cadillac converter . But fixed on a bank code an said nothing is covered under factory warranty.

    Few weeks earlier they supposedly put a water pump in our flex had to pull engine. But could not produce any of the old parts. Claimed they put new surprntine belt an all new radiator hoses.
    But steam cleaned engine an spray it done like they were going to put it on they’re used car lot.

    Claimed the back up camera could noy be replaced under the factory warranty. An did not fix it.
    Ford corporate office said it’s under warranty.
    This time they fixed camera . An instead of saying code came back to s mass air flow problem. An throttle body that was recalled an fix. Saying that if it came back to Cadillac converter an o2 sensors it would be covered.
    Now that’s not covered an the camera was covered. But they put a truck camera in it to be able to back up to s trailer with a basic line down the middle an now the camera an the vehicle d when you are backing up. The green is on the car behind you on parking lot.
    Ant they made us pay 800.00to fixed Cadillac converter but did not add the brand new O2 sensor on the invoice.
    Pulled code from auto zone an Barstow ford an the BAR with dmv an citrus service writer says we’re all gu of it. An he’d right.
    Now he’s telling me that the 100,000 mile warranty stoped at 80,000 for ford factory well only pay them for warranty work done on car up to 80,000 thousand. An our premium service contract ended at 76000 instead of 80000.

    We’ve been paying for work ever since paying cash for new car

    Sunland ford an now citrus are simply ripping they’re customers off saying the parts are not covered.
    Under factory unless it recall work. An they’re getting away with it.


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