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Servicing Oxygen Sensors in Farm Equipment

Due to more emission control standards, oxygen sensors can now be found in off-road uses such as the farm’s UTV. The terms oxygen sensor and lambda sensor are interchangeable. A diesel engine does not employ this device.

Every fuel has a stoichiometric value. It identifies the ratio of fuel-to-air for the most efficient combustion.

For pure gas, the stoichiometric value is nearly 14.7:1 (14 parts of air to 1 part of fuel). As the ratio goes numerically lower, the mixture is richer and the converse applies. Hybrid fuels such as E10 have a lower stoichiometric value since the combustion characteristics are fewer and the energy content of alcohols is lower than petroleum-based fuel. The following are approximate stoichiometric values of blends.

  • Pure gas 14.68:1 E10 14.08:1
  • E15 13.8:1 E85 9.85:1
  • Pure ethanol (E100) 9:1

When an engine is labeled as flex-fuel, it has an additional sensor in the fuel system that measures the ethanol content of the gas. The engine controller modifies the amount of fuel delivered to each cylinder via the injector to create the required stoichiometric value for that blend.

A gas engine is often equipped with a catalytic converter. (A catalyst is something that speeds up a chemical reaction without being consumed itself.) The purpose of the catalytic converter is to alter the exhaust gas from the engine into a benign form. This is called the conversion process. For the catalyst to have a high rate of conversion, two things need to happen: It must reach a minimum of 600°F. and the engine-out fuel mixture must be at or near stoichiometric. When this occurs, the catalyst is considered to be lit-off. The unit looks similar to a muffler but contains various precious metals in a substrate that resembles a honeycomb. 

There are numerous oxygen sensor designs based on the internal materials and other factors, but their purpose is the same. Their job is to act as an auditor and tell the engine controller if the mixture is at stoichiometric. If it is not, the injection system either leans or richens the air-fuel ratio to satisfy the oxygen sensor. 

The most commonly used oxygen sensor produces a minute voltage of between 0.100 volt and 0.900 volt. The voltage is created by the chemical reaction of the exhaust and the material in the sensor. When the mixture is rich, the sensor output is high (above 0.450 volt); when lean, it is low. Stoichiometric is around the midpoint of the voltage range. The sensor has a port that samples the oxygen content in the atmosphere and measures that proportionally to what is found in the exhaust gas. That is how it determines the mixture strength. 

The sensor is considered a consumable and needs to be replaced. As the sensor ages and is exposed to exhaust, engine oil and coolant (from a head or intake manifold gasket failure) skew the output. Then the sensor requires a richer mixture to produce the same voltage. Thus, it is telling the injection system to add fuel when the engine does not need or want it. At first, the only telltale sign is an increase in fuel consumption. As the sensor further degrades and adds fuel, it will result in diluting the engine oil with gas-washing oil from the cylinder walls. This can cause excessive wear to the engine bearings and piston rings along with glazing of the cylinder wall. 

A standard protocol is to allow 25% correction from the sensor. That would mean the engine can be running at an 11:1 ratio (extremely rich) and there will be no diagnostic codes in the system. Once the 25% threshold is passed, the mixture is considered out of control, and a trouble code would be stored. Many engines have been ruined by a degraded sensor.

The sensor lasts longest when the engine is tuned properly, ingests no coolant or oil, and isn’t extensively idled or incurring numerous cold and warm restarts. Since most sensors cost less than $100, it is a wise practice to regularly replace them. 

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