Ram 1500, 2500, 3500, 2WD & 4WD, 2001 - 2005

Heated Oxygen Sensor (HO2S)


Description & Operation

The oxygen sensor (O2) is a device that produces an electrical voltage when exposed to the oxygen present in the exhaust gases. The sensor is mounted in the exhaust manifold. Some oxygen sensors are electrically heated internally for faster switching when the engine is running. The oxygen sensor produces a voltage within 0 and 1 volt. When there is a large amount of oxygen present (lean mixture), the sensor produces a low voltage (less than 0.4v). When there is a lesser amount present (rich mixture) it produces a higher voltage (0.6-1.0v). The stoichiometric or correct fuel to air ratio will read between 0.4 and 0.6v. By monitoring the oxygen content and converting it to electrical voltage, the sensor acts as a rich-lean switch. The voltage is transmitted to the engine controller. The controller signals the power module to trigger the fuel injector.

Later models, have two sensors, one before the catalytic converter and one after. This is done for a catalyst efficiency monitor that is a part of the OBD-II engine controls that are on these year vehicles. The one before the catalyst measures the exhaust emissions right out of the engine, and sends the signal to the ECM about the state of the mixture as previously talked about. The second sensor reports the difference in the emissions after the exhaust gases have gone through the catalyst. This sensor reports to the ECM the amount of emissions reduction the catalyst is performing.

The oxygen sensor will not work until a predetermined temperature is reached, until this time the engine controller is running in what as known as OPEN LOOP operation. OPEN LOOP means that the engine controller has not yet begun to correct the air-to-fuel ratio by reading the oxygen sensor. After the engine comes to operating temperature, the engine controller will monitor the oxygen sensor and correct the air/fuel ratio from the sensors readings. This is what is known as CLOSED LOOP operation.

A heated oxygen sensor has a heating element that keeps the sensor at proper operating temperature during all operating modes. Maintaining correct sensor temperature at all times allows the system to enter into CLOSED LOOP operation sooner.

In CLOSED LOOP operation the engine controller monitors the sensor input (along with other inputs) and adjusts the injector pulse width accordingly. During OPEN LOOP operation the engine controller ignores the sensor input and adjusts the injector pulse to a preprogrammed value based on other inputs.

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The exhaust pipe gets extremely hot during engine operation, and if touched, severe burns can occur. If servicing the oxygen sensor, avoid contacting the exhaust system.

  1. Start the engine and bring it up to operating temperature.
  3. Raise and support the vehicle.
  5. Backprobe the O2 sensor between the O2 sensor output wire and ground with a suitable high impedance voltmeter.
  7. The O2 sensor should be rapidly switching between 0 and 1v. If working properly, it should be switching from a lean mixture (less than 0.4v) to a rich mixture (0.6-1.0v), and back. The average voltage should fall between 0.4-0.6v.
  9. If the sensor switches slowly, or is stuck in the middle of the range, the O2 may be faulty.
  11. If the sensor is stuck rich or lean, it most likely indicates a problem with the engine; for example, a vacuum leak would cause the O2 to read a lean mixture, and a malfunctioning fuel pressure regulator would cause a rich mixture.
  13. If the O2 sensor is above or below the specified range (0-1v), a wiring or computer problem is most likely the cause.
  15. Lower the vehicle. Turn the engine off.

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