1991 Mercedes-Benz 420SEL Oxygen Sensor Test Cost

Depending on the type of oxygen sensor in a vehicle, it may last 50-60K miles or more.

To diagnose a bad oxygen sensor, a technician will use a special scan tool to read the Diagnostic Trouble Code (DTC) responsible for the check engine light. If the code reveals that the oxygen sensor is at fault, the technician will determine (often by way of the scan tool ) which sensor is showing a fault. Since the scan tool does not diagnose whether or not the oxygen sensor is actually bad (only that there is a fault in the circuit), a technician will also use a vehicle repair manual and a multimeter to perform a pinpoint test aimed at verifying the status of the sensor. That pinpoint test will include locating the sensor, disconnecting the harness plug, turning the ignition "on", and checking the harness plug for battery voltage. If voltage is confirmed present, the technician will check the engine ground to the circuit and test the resistance (impedance) across the terminals of the sensor plug. The information gathered by this series of tests should tell the technician whether or not the sensor is working.

If an oxygen sensor is determined to be faulty, a technician may have to lift and support the vehicle for access to the sensor. The technician unplugs the sensor and uses a special wrench to remove it from its mount. This is often a difficult procedure because the constant heating and cooling of the exhaust system causes the sensor threads to seize. It is not uncommon for a sensor to break during removal. A new sensor is then installed in the mount on the exhaust pipe and connected to the wiring harness. The technician will lower the vehicle and start the engine to verify that the repair was successful. The DTC may also need to be cleared before the vehicle is put back into service.

RepairPal recommends replacing an oxygen sensor with the same type of sensor that is removed, an OEM or dealer-provided part. Aftermarket sensors are available at a reduced cost, but some vehicle computers may not recognize an aftermarket sensor and it may be impossible to clear the fault code from the system.

As with many repairs, the most challenging aspect is not the replacement procedure, but diagnosis of the problem. Many aftermarket auto parts stores offer free scanning of trouble codes (check engine light, etc.) with the goal of selling auto parts. But a DTC that points to an oxygen sensor may not be the result of a bad sensor, but rather a broken wire or other problem in the circuit. Further testing of the system and the sensor is necessary to pinpoint the cause.

Someone with an intermediate level of DIY experience can replace a bad oxygen sensor as long as the sensor has been properly diagnosed. If the old sensor breaks during removal, a more complex procedure will be necessary. It may also be necessary to have the DTC cleared.