How to Test O2 Sensor with Multimeter?

Basics to the o2 probe

The o2 sensor sends important signals to the engine management system to control the fuel mixture composition by determining the residual oxygen content in the exhaust gas. For this purpose, the oxygen content in the exhaust gases is compared with the oxygen content of the current ambient air. The ratio of combustion air to fuel is then determined from the two measurement results.

The optimum combustion air ratio consists of 14.7 kg of air to 1 kg of fuel and is referred to as a stoichiometric mixture. It is abbreviated to the Greek letter λ (02).

The residual oxygen content of the exhaust gases (about 0.3 – 3 percent) is compared with the oxygen content of the ambient air (about 20.8 percent). If the residual oxygen content of the exhaust gas corresponds to 3 percent, this is referred to as a lean mixture. Due to the difference in the oxygen content of the ambient air creates a voltage of 0.1 volts. If the residual oxygen content of the exhaust gas is less than 3 percent, this is called a rich mixture. The voltage increases in the ratio of the increased difference to 0.9 volts.

Two different measuring methods are used with the o2 probe:

• Surge probe (Nernst probe) with a conductive zirconium dioxide membrane, where the different oxygen concentration on both sides of the probe generates a measurable voltage.

• Resistance-jump probe with a semiconducting titanium dioxide ceramic, in which the measured values are determined by the voltage drop in the resistor combination.

Os sensors have been used since the 1970s mainly in gasoline engines in vehicles. Originally, the o2 probe was installed in the exhaust manifold. Since the introduction of current EOBD guidelines, the function of the catalytic converter must also be monitored, which is why modern vehicles are generally equipped with several o2 sensors.

O2 sensors are not only found in gasoline engines and diesel engines in vehicles, but also in the field of home heating technology, for example in wood chips, biomass, or pellet heaters.

O2 sensors Defects and their causes

Although the o2 probe is relatively robust as a self-contained component, defects can occur. In the case of suspected malfunction, the o2 probe should be checked.

The following symptoms may indicate a defect in the o2 probe:

• Higher fuel consumption
• Worse engine performance
• Higher exhaust emissions during exhaust emission test (AU)
• Lighting up the engine control lamp
• Display of an error code

The following causes can be responsible for a defective o2 probe:

• Short circuits of the electronics inside the probe
• Short circuits of the electronics outside the probe
• Missing power supply due to dirty or defective contacts
• Missing ground supply due to dirty or defective contacts
• Overheating of the o2 probe
• Deposits or soiling on the probe
• Mechanical damage
• Use of incompatible fuel additives

Check o2 probe – instructions

If a defect is suspected, the o2 probe should be checked.

Car repair shops have several options:

• Check o2 probe with exhaust gas tester
• Check o2 probe with an o2 probe

But it does not always have to be an expensive workshop visit.
You can also use a multimeter to test the o2 probe yourself.

1. Visually check o2 probe

Before carrying out an electronic test of the o2 probe with the multimeter, however, the condition of the protective tube housing should first be visually checked. Already here, any problems can be identified and appropriate countermeasures initiated:

• Very sooty protective tube

A heavily sooted thermowell indicates the burning of too rich a mixture. In this case, the o2 probe must be replaced and readjusted to restore an optimal combustion air ratio and prevent recrudescence.

• Deposits on the protective tube

Deposits on the white protective tube indicate problems with the ingredients of the fuel used. For example, light deposits may occur when the engine burns oil, or incompatible additives are present in the fuel. In this case, the probe must be renewed entirely and the cause of the deposits repaired.

• Damage to the protective tube

Damage to the white protective tube may result in the improper installation of the o2 probe, which may impair its functionality. This may be the case if the specified special tool was not used or the tightening torque was not respected. The probe should be professionally replaced.

If no external signs indicate a defect, an electronic check can be carried out by measuring the o2 probe with the multimeter.

2. Check the o2 probe with a multimeter

For this procedure, only a modern, fast-displaying multimeter with measuring leads is needed.

The multimeter is shot against the ground to the o2 probe.

To do this, the red measuring line of the multimeter is connected in parallel to the signal line of the o2 probe (black cable). The black test lead of the multimeter is grounded to the housing of the engine compartment.

The measuring range of the multimeter should be set to 1 or 2 volts.

After switching on the ignition while the engine and probe are still cold, a basic voltage of about 450 mV should be applied.

When the motor or probe reaches operating temperature, the readings should oscillate between 0.1 and 0.9 volts.

This suggests the proper function and an o2 value of 1. To obtain a meaningful measurement result, the engine should be kept at about 2500 rpm. In this way, the optimum operating temperature is reached.

In the case of o2 sensors with heating, the internal resistance and the voltage supply of the heating element can also be measured with the multimeter.

For this purpose, the plug must be disconnected from the o2 probe.

O2 probe side can be measured with the multimeter on the two cables of the heating element, the resistance. This should be between 2 and 14 ohms. On the vehicle side, the voltage supply can be measured with the multimeter. This should be about 10 volts.

Basic connection options and cable colors for the o2 probe

Unheated probes
1 black cable – signal (ground across housing)

or

2 black cables – signal and ground

Heated probes
1 black cable signal (ground across housing) 
2 white cables – heating element

or

2 black cables – signal and ground 
2 white cables – heating element

General note: Before applying the measuring instrument, always observe the manufacturer-specific information in the vehicle owner’s manual!