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Pelican Technical Article:

Oxygen Sensor Replacement

Nick Czerula

Time:

2 hours2 hrs

Tab:

$300

Talent:

****

Tools:

Oxygen sensor socket, 22mm wrench, flathead screwdriver, 8mm nut driver, torque wrench, floor jack, two jack stands, two wheel chocks, safety glasses

Applicable Models:

Mercedes-Benz W211 (2003-09)

Parts Required:

Oxygen sensors, anti-seize paste

Hot Tip:

Work with a cool exhaust

Performance Gain:

Remedy fault codes and maintain a proper running engine

Complementary Modification:

Replace in pairs

Automotive oxygen sensors, more commonly known as O2 sensors, make fuel injection and emission control possible. An oxygen sensor reports to the DME if the air-fuel ratio is rich or lean. Since oxygen sensors are located in the exhaust stream, they do not directly measure the air or the fuel entering the engine but when information from oxygen sensors is coupled with information from other sources, it can be used to indirectly determine the air-fuel ratio. The sensor doesn't measure oxygen concentration, oddly as it seems. The sensor measures the difference between the oxygen in the exhaust gas and oxygen in ambient air. A rich exhaust mixture causes an oxygen demand. This demand causes a voltage build up, due to transportation of oxygen ions through the Zirconia sensor layer. A lean mixture causes low voltage, since there is an oxygen excess.

Oxygen sensors should be replaced every 100,000 miles. In a perfect world that would be it; wait until a specified mileage and replace the sensor. However, these sensors fail prematurely, set oxygen sensor fault codes and reduce fuel economy. In this article, I will show you how to identify the location of all four oxygen sensors and how to replace them. You will need an oxygen sensor socket to remove the sensors and remember to always work with a cool exhaust. Oxygen sensors are fragile. Do not drop, as damage may occur. Also, keep the sensor tip clean when reinstalling the sensor.

Oxygen sensors are laid out in banks, which refer to cylinders and in sensor numbers referring to before or after the catalytic converter. Bank1 or B1 refers to cylinders 1-3. Bank 2 or B2 refers to cylinders 4-6. Sensor 1 or S1 refers to sensor before catalytic converter. Sensor 2 or S2 refers to the sensor after the catalytic converter.

Remember, your car may have been serviced before and parts replaced with different size fasteners used in the replacement. The sizes of the nuts and bolts we give may be different from what you have so be prepared with different size sockets and wrenches.

Protect your eyes, hands and body from fluids, dust and debris while working on your vehicle. If working with the electrical system, disconnect the battery before beginning. Always catch fluids in appropriate containers and properly dispose of any fluid waste. Recycle parts, packaging and fluids when possible. Never work on your vehicle if you feel the task is beyond your ability.

Safely jack up the front of your M-B. Make sure to consult our article on this subject should you need assistance.

Remove the lower engine splash shields. See our tech article on engine splash shield removing.

Sensors: Bank 1 Sensor 1, blue arrow.
Figure 1

Sensors: Bank 1 Sensor 1, blue arrow. Bank 2 Sensor 1, red arrow. Bank 1 Sensor 2, green arrow. Bank 2 Sensor 2, yellow arrow.

Sensor connectors: Bank 1 Sensor 1, red arrow.
Figure 2

Sensor connectors: Bank 1 Sensor 1, red arrow. Bank 2 Sensor 1, green arrow. Bank 1 Sensor 2, blue arrow. Bank 2 Sensor 2, yellow arrow.

Replacing sensors before catalytic converter: Oxygen sensors are located in the exhaust front pipe.
Figure 3

Replacing sensors before catalytic converter: Oxygen sensors are located in the exhaust front pipe. Label or mark the wiring harness to aide during reinstallation. I suggest replacing one sensor at a time so you do not mix up the connectors. Pull the wiring harness out of the mount. Then disconnect the electrical connector by pulling it straight apart (inset).

Replacing sensors before catalytic converter: Using an oxygen sensor socket, loosen the oxygen sensor connection to the exhaust manifold.
Figure 4

Replacing sensors before catalytic converter: Using an oxygen sensor socket, loosen the oxygen sensor connection to the exhaust manifold. Do not unscrew it yet. Follow the harness up around the exhaust. Cut or remove any zip ties securing the harness to the engine. Using an oxygen sensor socket, remove the oxygen sensor from the exhaust. Lightly coat the new oxygen sensor thread with anti-seize compound. Install the new oxygen sensor and torque it to 50Nm (37 ft-lb). Then reroute the wiring harness and connect the electrical connector. Repeat this process if you're replacing both of the sensors. Then reassemble the engine covers and clear any engine fault codes using a Mercedes-Benz scan tool. When reinstalling the sensor, do not allow the harness to remain twisted. Be sure it is properly routed and tension free. And replace the zip ties you previously cut.

Replacing sensors after catalytic converter: The oxygen sensors are located in the exhaust, behind the catalytic converters.
Figure 5

Replacing sensors after catalytic converter: The oxygen sensors are located in the exhaust, behind the catalytic converters. Label or mark the wiring harness to aide during reinstallation. I suggest replacing one sensor at a time so you do not mix up the connectors. Pull the wiring harness out of the mount. Then disconnect the electrical connector by pulling it straight apart (inset).

Replacing sensors after catalytic converter: Using an oxygen sensor socket, loosen the oxygen sensor connection to the exhaust manifold.
Figure 6

Replacing sensors after catalytic converter: Using an oxygen sensor socket, loosen the oxygen sensor connection to the exhaust manifold.

Replacing sensors after catalytic converter: Remove the oxygen sensor from the exhaust.
Figure 7

Replacing sensors after catalytic converter: Remove the oxygen sensor from the exhaust. Lightly coat the new oxygen sensor thread with anti-seize compound. Install the new oxygen sensor and torque it to 50Nm (37 ft-lb).

Then reroute the wiring harness and connect the electrical connector.
Figure 8

Then reroute the wiring harness and connect the electrical connector. Repeat the same process if you are replacing both sensors. Then reassemble the engine covers and clear any engine fault codes using a BMW scan tool.

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