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Pelican Technical Article:

Replacing the Oxygen Sensor

Mike Holloway


10 minutes10 mins






10mm wrench, 10mm hex socket, 14mm Allen socket, 10 inch extension, universal swivel, floor jack, jask stands, wheel chocks, safety glasses, torque wrench

Applicable Models:

Mercedes-Benz R107 (1975-86)

Parts Required:

Oxygen sensor Pelican Part #11027-M14

Hot Tip:

Look to ground the sensor housing

Performance Gain:

Better temperature and fuel management

Complementary Modification:

Longer life of the engine, better fuel economy

Before you do any work on your car it is important that you wear safety glasses and work gloves. If you have to jack up your car, make sure to use jack stands and chock your wheels as well as applying the parking brake. Protect your eyes, hands and body from fluids, dust and debris while working on your vehicle. Never work on your vehicle if you feel the task is beyond your ability. Always disconnect the battery before working on your car.

The oxygen sensor is a very easy item to replace and is one of the most important parts on your car. It should be replaced every 30,000 miles. In 1976, Bosch introduced what many believe to be one of the most important technologies for reducing exhaust emissions: the oxygen sensor. It has been written that oxygen sensor failure is the "single greatest source of excessive emissions for fuel-injected vehicles" and the second most significant cause of high emissions in carbureted engines. The U.S. Environmental Protection Agency (EPA) and the California Air Resources Board (CARB) have found that oxygen sensor replacement was required on almost 60% of all vehicles.

Speaking with a representative from Bosch, I was informed that the oxygen sensor was called the "Lambda Sensor" and it was first used in fuel-injected European cars. The oxygen sensor monitors the level of oxygen in the exhaust. The onboard computer regulates the air/fuel mixture to reduce emissions. The ratio for the 450SL is 14.7:1. The sensor is mounted in the exhaust manifold downpipe(s) before the catalytic converter or between the exhaust manifold and the catalytic converter. It generates a voltage signal proportional to the amount of oxygen in the exhaust. The sensing element on nearly all oxygen sensors in use is a zirconium ceramic bulb coated on both sides with a thin layer of platinum. The outside of the bulb is exposed to the hot exhaust gases, while the inside of the bulb is vented internally through the sensor body or wiring to the outside atmosphere.

When the air/fuel mixture is rich and there is little oxygen in the exhaust, the difference in oxygen levels across the sensing element generates a voltage through the sensor's platinum electrodes, which is between 0.8 to 0.9 volts. When the air/fuel mixture is lean and there is more oxygen in the exhaust, the sensor's voltage output drops to 0.1 to 0.3 volts. When the air/fuel mixture is perfectly balanced and combustion is cleanest, the sensor's output voltage is around 0.45 volts. When the computer sees a rich signal (high voltage) from the oxygen sensor, it tells the fuel mixture to go lean. When it receives a lean signal (low voltage) from the oxygen sensor, it tells the fuel mixture to go rich. Cycling back and forth from rich to lean averages out the overall air/fuel mixture to minimize emissions and to help the catalytic converter operate at peak efficiency, which is necessary to reduce hydrocarbon (HC), carbon monoxide (CO) and oxides of nitrogen (NOX) levels even further.

The normal aging process will eventually cause the oxygen sensor to fail. However, the sensor may also fail prematurely if it becomes contaminated with phosphorus from excessive oil consumption or silicone from internal coolant leaks or using silicone sprays or gasket sealers on the engine. Environmental factors such as road splash, salt, oil and dirt can also cause a sensor to fail, as can mechanical stress or mishandling. A dead sensor will prevent the onboard computer from making the necessary air/fuel corrections, causing the air/fuel mixture to run rich in the "open loop" mode of operation, resulting in much higher fuel consumption and emissions. The failure of the oxygen sensor may be damage to the catalytic converter. If the signal is faulty, the engine will run too rich making the converter run hotter than normal. If the converter gets hot enough, the catalyst substrate inside may actually melt forming a partial or complete blockage. The result can be a drastic drop in highway performance or stalling because of a buildup of backpressure in the exhaust system.

Replacing the oxygen sensor will not only reduce emissions but will provide proper combustion efficiency, which may save you up to 15% in fuel costs. Cars built from the mid 70's to the mid 80's should have the oxygen sensor replaced every 30,000 miles. Grounding the oxygen sensor body may help reduce CO caused by the engine running rich. Cars built in the mid 80's use ground straps on the sensors to eliminate the electrical resistance problem in aging exhaust systems. These are four-wire systems. You may notice that your oxygen sensor has one or three leads. Three wire sensors still have only one signal wire, the other two wires are for the heater, and do not affect the ground electrical resistance, which is still through the exhaust pipe.

It is a really easy replacement job. While you are at it, you may also want to ground the sensor. This is an easy job as well. Just screw a small hose clamp to the sensor and fix a braided cable to the clamp and the screw holding the sensor cable. Before you do that you will want to check the resistance of the sensor. You can do this by checking the resistance using a multi-meter. Check the resistance of the sensor - anything under 10 ohms and you should be fine. Put a ground strap on if you are getting over 1.5 K ohms between the sensor body and the chassis ground.

The oxygen sensor is located on the exhaust line.
Figure 1

The oxygen sensor is located on the exhaust line. 

Using a 22mm open end wrench or a crowsfoot wrench on a socket wrench, loosen and remove the sensor.
Figure 2

Using a 22mm open end wrench or a crowsfoot wrench on a socket wrench, loosen and remove the sensor. 
Companies such as Bosch suggest that oxygen sensor performance can be checked by reading the sensor's output voltage to make sure it corresponds with the air/fuel mixture (low when lean, high when rich). The voltage signal can also be displayed as a wave form on an oscilloscope to make sure the signal is changing back and forth from rich to lean and is responding quickly enough to changes in the air/fuel ratio. You may not have an oscilloscope or even know how to operate one. A voltmeter will do the trick.

Place one lead on the chassis or transmission and one on the oxygen sensor housing.
Figure 3

Place one lead on the chassis or transmission and one on the oxygen sensor housing. Turn your meter to read ohms (resitance). If your reading is under 10 ohms, the sensor is fine but anything over 10 ohms would suggest that you will want to ground the sensor. Use an engine ground strap and fix it to the sensor housing using a hose clamp. The cable can be attached to the frame. 

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Comments and Suggestions:
Tor Comments: Hello.
I have a Mercedes SL 450 1975 6/75 GVWR 4600 VIN 107044 2 027I24 Model 107
There is a problem with hot exhaust manifold right side see picture tube is glowing.
Have been looking for oxygen sensor, but I can not find it?
The engine runs rough.
Possible bad fuel injector also?
Possible to get any help?

Thank you in advance.

Best regards
August 30, 2016
  Followup from the Pelican Staff: If the manifold is glowing it is due to a misfire. I would check that cylinder for spark, fuel and compression. - Nick at Pelican Parts  

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