P0131: O2 Sensor Circuit Low Voltage (Bank 1 Sensor 1)

In this section we're gonna' take a look at what code P0131 really means and in the following heading (on this same page), I'm gonna' show you how to troubleshoot it.

OK, in plain English, a P0131 O2 Sensor Circuit Low Voltage (Bank 1 Sensor 1) diagnostic trouble code (DTC) is telling you:

  1. That the upstream oxygen sensor (located before the catalytic converter) is reporting a low voltage to the PCM (Powertrain Control Module). This continuous voltage will be a value under .5 Volts (500 millivolts).
  2. To be a bit more specific, this ‘stuck’ voltage will be a value between .1 Volt to .5 Volts (100 to 500 millivolts).
  3. Normally, any continuous voltage, from the upstream O2 sensor, below .5 Volts indicates a Lean condition... but not always, since the O2 sensor can fail and produce the same result (although a Lean condition does not really exists)
    1. A Lean condition simply refers to the fact that the engine's air/fuel mixture is too oxygen heavy (in other words, not enough fuel is present for proper/ideal combustion).

Remember, in a normally operating engine, the front (upstream) O2 sensor will produce a voltage that will constantly change between .1 Volt to .9 Volts DC. When a P0131 pops up, the PCM is seeing a voltage that stays continually fixed somewhere below .5 Volts. This could be caused by a:

  1. BAD oxygen sensor.
    1. The oxygen sensor fails internally in such a way that it stays stuck producing a continuous low voltage. This low voltage will be something below .5 Volts (500 millivolts).
  2. A lean condition due to a:
    1. Failing fuel pump.
    2. Dirty fuel filter.
    3. Clogged fuel injectors.
    4. Vacuum leak from bad intake manifold gaskets (these are made of plastic) or a vacuum hose(s).

In the next section I'll show you how to diagnose this trouble code and find out if the upstream (bank 1 sensor 1) oxygen sensor is the one behind it.

How To Diagnose Trouble Code P0131

How To Test The Oxygen Sensor (GM 3.8L)

If your 3.8L equipped GM vehicle is registering a P0131 diagnostic trouble code... the first thing you'll need to do is connect a scan tool with live data mode capability and confirm that the front (upstream) O2 sensor voltage is really stuck below .5 Volts.

Once you've made sure that the upstream O2 sensor is stuck producing a low voltage (by seeing this ‘stuck’ voltage on the scan tool), you can easily test the oxygen sensor by inducing a rich condition with carburetor cleaner.

When a rich condition is induced on a running engine... the upstream (pre-catalytic converter) oxygen sensor's voltage will shoot up to .9 Volts (900 millivolts).

If the oxygen sensor responds with a higher voltage (when you spray the carb. cleaner)... then this test result will tell you that the O2 sensor is good and operating properly and that the problem is elsewhere.

This is what you'll need to do:

  1. 1

    Connect your scan tool to the your car or mini-van and start the engine (Don't have a scan tool? Need a scan tool? check out my recommendation: Actron CP9580 scan tool Review).

    Let the engine idle for at least 15 minutes before you start the test, to get the O2 sensor to activate.

  2. 2

    Now, on your scan tool (and once you're in Live Data mode), scroll down to the PID that's labeled O2S11. This PID is the one that will show you what the oxygen sensor is reporting in Volts DC.

  3. 3

    Now, take a look at the voltage readings for O2S11.

    These should be constantly moving between any number between 0.100 Volts and 0.900 Volts.

    If the voltages are not moving between 0.100 and 0.900 Volts, don't worry about it just yet... let's go on to the next step.

  4. 4

    With the engine running, spray a little carburetor cleaner into a vacuum hose (that has engine vacuum) while you observe your scan tool's display screen.

    You're not going to be able to spray carb cleaner into the throttle as the engine is running, because if you were to do this, the engine will die as soon as you disconnect the air duct from it (to spray into it).

    If you spray too much, the engine will stall. If this happens to you, just restart the engine and repeat the step and spray less carb cleaner spray.

  5. 5

    As you spray some short burst of carb cleaner into the vacuum hose, you should see the voltage numbers of O2S11 immediately spike to 0.800 to 0.900 Volts. And as long as you're spraying, these voltage number should stay there.

  6. 6

    When you stop spraying, the O2 sensor values should come down and within a few seconds, they should start oscillating between 0.100 Volts to 0.900 Volts. If they don't... don't worry about it yet.

Let's take a look at what your test results mean:


CASE 1: The upstream O2 sensor's voltage shot up to .9 Volts This tells you that the O2 sensor is operating normally and that the problem causing the P0131 is elsewhere.

You'll need to test to see if the P0131 is being caused by:

  1. A lean condition due to a:
    1. Failing fuel pump.
    2. Dirty fuel filter.
    3. Clogged fuel injectors.
    4. Vacuum leak from bad intake manifold gaskets (these are made of plastic) or a vacuum hose(s).

CASE 2: The upstream O2 sensor's voltage DID NOT shoot up to .9 Volts This tells you that the upstream oxygen sensor is bad and needs to be replaced.

If the oxygen sensor was operating normally, it would have reacted to the carburetor cleaner immediately by producing a voltage around .9 Volts (900 millivolts)... since it did not, you can deduce correctly that it's fried and needs to be replaced.