How To Test Trouble Code P0131 (Dodge 3.9L, 5.2L, 5.9L)

Testing a P0131 O2 Sensor Circuit Low Voltage (Bank 1 Sensor 1) is not hard.

In this tutorial, I'm gonna' show you some of the basics you need to know about what DTC P0131 means and a step-by-step test of the upstream oxygen sensor.

Although this tutorial is geared toward a 3.9L, 5.2L, or 5.9L Dodge/Jeep pick up, van, or SUV, the info and test can be used on any 1996-2003 make or model.

P0131 Basics You Need To Know

Before we jump into diagnosing the P0131 Diagnostic Trouble Code (DTC) that's lighting up your check engine light (CEL), in this section I'll share with you some basic, but important information.

LOCATION: The oxygen sensor, that trouble code P0131 is referring to, is located before the catalytic converter and is known by several different names. The most common are:

1. Upstream oxygen sensor.
2. O2S11.
3. Pre-catalytic converter oxygen sensor.
4. Front O2 sensor.

Some vehicles have only one upstream oxygen sensor and some have two upstream oxygen sensors. Here are some more specifics:

1. On vehicles with two upstream oxygen sensors, a P0131 DTC is identifying the one that's on Bank 1. Bank 1 is the bank of cylinders that have cylinders 1, 3, 5, and 7 (5.2L and 5.9L V-8 engines) and 1, 3, and 5 (3.9L V-6 engines).
2. On vehicles with one upstream oxygen sensors, a P0131 DTC is identifying the only one that's before the catalytic converter.

DEFINITION: P0131 O2 Sensor Circuit Low Voltage (Bank 1 Sensor 1) diagnostic trouble code (DTC) is telling you:

1. That the oxygen sensor, located before the catalytic converter, is reporting a continuous low voltage to the PCM (Powertrain Control Module). This continuous voltage will be a value under 0.5 Volts (500 millivolts).
2. This continuous low voltage is being reported by the upstream O2 sensor for more than 2 minutes.
3. This continuous low voltage will be a voltage value between 0.1 Volt to 0.5 Volts (100 to 500 millivolts).
4. Normally, any continuous voltage, from the upstream O2 sensor, below 0.5 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)
   • 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). This is usually due to a large vacuum leak, or a bad fuel pump, etc.

OPERATION: In a normally operating engine, the front (upstream) O2 sensors will produce a voltage that will constantly and rapidly change between 0.1 Volt to 0.9 Volts DC.

So, when a P0131 pops up, the PCM is seeing a voltage that stays continually fixed somewhere below 0.5 Volts for more than 2 minutes.

PROBABLE CAUSES: Several things can cause a P0131 diagnostic trouble code, the most common causes of a P0131 DTC are:

1. Bad oxygen sensor.
   • This is probably the most common cause of a P0131 DTC.
   • 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 0.5 Volts (500 millivolts).
2. A lean condition due to a:
   • Failing fuel pump.
   • Dirty fuel filter.
   • Clogged fuel injectors.
   • Vacuum leak from bad intake manifold gaskets or a vacuum hose(s).
3. PCM not getting the correct signal due to a short in the O2 sensor's wiring.
   • This usually the end result of the O2 sensor's wiring or the engine wiring harness coming into contact with the exhaust pipe, exhaust manifold or a hard edge (usually on the engine) that causes wiring's insulation starts to melt.
4. Bad PCM. This is extremely rare.

With this basic info we can now start troubleshooting the P0131 DTC, let's turn the page and get testing.