Oxygen Sensor: Testing and Inspection
Oxygen Sensors Functional Check
Introduction
In vehicles with a diesel engine, up to two oxygen sensors (depending on the version) are installed to comply with exhaust emission standards.
In vehicles with a single oxygen sensor, the oxygen sensor is installed upstream of the oxidation catalytic converter.
In vehicles with two oxygen sensors, one oxygen sensor is installed upstream of the oxidation catalytic converter and the second is installed downstream of the oxidation catalytic converter. Both oxygen sensors are identical.
The function of the oxygen sensors can be assessed by evaluating the air ratios in different operating conditions.
Preconditions
- Oxygen sensor operating readiness
The oxygen sensors can only emit a valid signal when at operating temperature and when the engine is running (after reaching the end of the dew point temperature). The oxygen sensor upstream of the oxidation catalytic converter reaches operating readiness when coolant temperature is between 70 °C and 80 °C.
The oxygen sensor downstream of the catalytic converter needs a minutes more than the upstream oxygen sensor to achieve operating readiness.
NOTE: Even if the engine is already at operating temperature when it is started, it still takes a few minutes for the oxygen sensors to become operational.
- Stored faults
If electrical faults are stored for the oxygen sensors, operating readiness is not granted and the functional check is blocked. In this case the electrical faults must be remedied first.
Plausibility criteria
The following criteria are important for checking the plausibility of air ratios:
1. When in a steady state, the air ratios upstream and downstream of the oxidation catalytic converter are virtually identical.
2. The air ratios fluctuate within the ranges shown in the examples.
3. The signal from the oxygen sensor downstream of the oxidation catalytic converter follows the signal from the upstream oxygen sensor after a certain time delay (approximately one second).
4. Erratic changes to the air ratio are signalled virtually immediately by the oxygen sensors.
NOTE: Heavy fouling of the upstream oxygen sensor can mean that its signals are delayed by up to two seconds. In this situation, the signal from the oxygen sensor upstream of the oxidation catalytic converter is sent after the signal from the downstream oxygen sensor.
Functional check in idle
During a functional check while idling, the air ratios are checked on the basis of their Behavior under different marginal conditions.
When idling, the air ratios are measured in a range from 1.5 to 8.
The following marginal conditions affect the air ratios:
- Exhaust-gas recirculation:
When exhaust-gas recirculation is active, the proportion of fresh air during combustion is lower, leading to a lower air ratio.
The exhaust-gas recirculation can be switched off for the purpose of the test, e.g. be entering a long continuous idle phase.
- Activating consumers:
Activating consumers increases the fuel quantity during combustion, leading to a lower air ratio.
The following graphic shows how marginal conditions can affect the air ratios when idling.
Functional check when pressing on accelerator
When carrying out a functional check while pressing on the accelerator (quick and complete operation of the accelerator pedal module), the dynamic Behavior of the air ratios is checked. Here, the air ratios are measured in a range from 1 to 32.
The air ratios behave as follows when the accelerator is pressed:
1. Operating the accelerator pedal module:
The engine acceleration and corresponding increase in fuel quantity lowers the air ratios.
2. Keeping the accelerator pedal module pressed:
Keeping the accelerator pedal module pressed (engine runs at maximum speed) stabilizes the air ratio at level considerably higher than the idle level.
3. Releasing the accelerator pedal module:
The fuel injection is switched off until the engine returns to idle speed. During this phase, the air ratios increase to a maximum value of 32.7 until the idle speed is reached and fuel injection is resumed.
The following graphic shows how the air ratios can behave when the accelerator is pressed.
NOTE: When exhaust-gas recirculation is switched off, the air ratios during the idle phase will be somewhat higher than those shown in this example.
