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Old October 13th, 2011, 10:56 AM
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The Wrong Signal to the Engine Computer

Oxygen sensors let the engine computer monitor the amount of oxygen in the exhaust. Increasingly complex oxygen sensors have been used on cars since the early 1980s. Some new cars have numerous O2 sensors.

When oxygen sensors fail or degrade, the engine computer adjusts the fuel/air ratio to be too rich or too lean. The engine might hesitate, stall, surge or otherwise run badly. The catalytic converter might be damaged. Gas mileage worsens. Failed emissions tests with “high CO and/or HC” can indicate oxygen sensor problems.

Oxygen sensors thread into sockets in the exhaust so they are exposed to a very hot, harsh environment and eventually will fail no matter how well the car is maintained. But there are things you can do to avoid prematurely damaging oxygen sensors. Silicone, engine oil, and lead in the exhaust gas will coat and contaminate the oxygen sensor probe . Maintain the vehicle to protect oxygen sensors from those three.

Some engine coolants contain silicone. Fix leaking head gaskets so coolant does not seep into the engine. Since the early 1990s, there have been manufacturer technical service bulletins with titles like "Oxygen Sensor Contamination Due to Silicone”. Using tubes of silicone (RTV) sealers on or instead of engine gaskets (valve cover gaskets, oil pan gaskets, etc.) introduces silicone into the engine exhaust that can damage the oxygen sensor probe.

Keep engine oil out of the exhaust as much as possible by replacing valve stem seals, piston rings, etc. Folks have reported that engine oil contamination on the OUTSIDE of an oxygen sensor can make the oxygen sensor send the wrong signal to the engine computer. Fix external oil leaks from valve cover gaskets, PCV valve grommets, etc. Trying to weatherize an oxygen sensor by covering or coating it with anything; silicone, tubing, grease, etc. is a bad idea.

Do not use leaded gas (aviation fuel and still sold at gas stations in some countries) and avoid fuel additives or at least make sure the label on the bottle says the contents are compatible with oxygen sensors.

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