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O2 Sensor
It's got a lot of names, an EGO sensor, Lambda sensor, Lambda sond, Lambda probe,O2 sensor, O2S, HO2S, and oxygen sensor, but they all do the same thing; sense the amount of oxygen in the exhaust.
Before an oxygen sensor can report accurately, it has to be warmed up. Anything below 575° Fahrenheit, is out of it's operating range. That's why many new oxygen sensors have built-in heaters. Aside from helping bring the oxygen sensor up to temperature sooner during warm-up, the heaters counteract the cooling effects of prolonged idling. Heating the oxygen sensor also keeps it cleaner, which extends its life. Electricity powers the heater. This is why modern oxygen sensors have three or four wires. On 4 wire oxygen sensors, the wires are; oxygen sensor output, oxygen sensor ground, 12 volt positive for the heater, and heater ground. On 3 wire oxygen sensors, the heater ground is fed through the case.
After it's reach operating temperature, oxygen sensors produce voltage in a range of zero to one volt. The amount of voltage it produces depends on the amount of oxygen in the exhaust stream.
The atmosphere is about 21% oxygen. A car's exhaust can be up to 3% oxygen. If the exhaust has 3% oxygen, the engine is running lean. Under these circumstances, the oxygen sensor will produce low voltage - something below 0.3 volts. If the exhaust has 0% oxygen, the engine is running rich. In this situation, the oxygen sensor will produce something above 0.6 volts. The ECU (electronic control unit) reads the voltage from the oxygen sensor, and when in closed loop mode, adjusts the air/fuel ratio based on that voltage. It's common for the voltage on an oxygen sensor to constantly switch back and forth between the 0.45 volt rich/lean point.
ECU's are usually in one of three modes: warm-up, open loop, and closed loop. The oxygen sensor is only used by the ECU while in closed loop mode. ECUs go into closed loop mode when the coolant temperature sensor indicates the engine is warmed up, AND when either at idle or at cruise.
If an oxygen sensor always reads lean, it may be worn out. They have a limited lifespan, and typically will produce low voltage numbers when dying. A number of other things can cause low voltage as well.
- The output wire insulation may be bad causing it to ground. A ground will cause a false lean signal.
- The MAP sensor may be bad and producing faulty vacuum to voltage output. A high vacuum signal will cause the ECU to assume the engine is at idle and attempt to run lean.
- Clogged injectors can appear as a lean condition.
- Low fuel pressure will cause lean conditions at any rpm or load range.
- Upstream exhaust leaks can pull air in.
- The air injection system should not direct air to the exhaust ports during closed loop operation.
Here are some things to that can cause an oxygen sensor to always reads rich:
- High fuel pressure.
- Injectors with poor spray patterns.
- A bad canister purge system can very dump large amounts of fuel into the intake stream. This can be checked by disconnecting the vapor hose.
- The MAP sensor may be bad and producing faulty vacuum to voltage output. A low vacuum signal will cause the ECU to assume the engine is under a load and attempt to run rich.
- A bad throttle position sensor can cause the system to go rich if the ECU sees a high signal.
Lastly, ignoring a dead oxygen sensor is a very bad idea. Dead oxygen sensors don't supply correct voltage levels. When the ECU sees low voltage it increases the injector pulse width, causing the engine to run rich. This excess gas increases the hydrocarbons in the exhaust, causing the catalytic converters to overheat and die. If you think a $100 oxygen sensor is a lot of money, wait until your Mercedes-Benz service writer tells you what the dead cats will cost.