Oxygen Sensor: Description and Operation

Fig. 17 Front Heated Oxygen Sensor:

Fig. 18 Rear Heated Oxygen Sensor:

HEATED OXYGEN SENSORS - PCM INPUT
The input from the upstream heated oxygen sensor tells the PCM the oxygen content of the exhaust gas. Based on this input, the PCM fine tunes the air-fuel ratio by adjusting injector pulse width.

The sensor input switches from 0 to 1 volt, depending upon the oxygen content of the exhaust gas in the exhaust manifold. When a large amount of oxygen is present (caused by a lean air-fuel mixture), the sensor produces voltage as low as 0.1 volt. When there is a lesser amount of oxygen present (rich air-fuel mixture) the sensor produces a voltage as high as 1.0 volt. By monitoring the oxygen content and converting it to electrical voltage, the sensor acts as a rich-lean switch.

The heating element in the sensor provides heat to the sensor ceramic element. Heating the sensor allows the system to enter into closed loop operation sooner. Also, it allows the system to remain in closed loop operation during periods of extended idle.

In Closed Loop, the PCM adjusts injector pulse width based on the upstream heated oxygen sensor input along with other inputs. In Open Loop, the PCM adjusts injector pulse width based on preprogrammed (fixed) values and inputs from other sensors.

The upstream oxygen sensor threads into the outlet flange of the exhaust manifold Fig. 17 and 18.

CIRCUIT OPERATION
Circuit A14 is connected to the BUS bar in the Power Distribution Center (PDC), which connects to battery voltage. The contact side of the Automatic Shut-Down (ASD) relay connects circuit A14 and circuit A142. A 20 amp fuse, located in cavity 5 of the PDC, protects circuits A14 and A142.

Circuit A14 also supplies voltage to the coil side of the ASD relay. The Powertrain Control Module (PCM) controls the ground path circuit for the coil side of the ASD relay on circuit K51. Circuit K51 connects to cavity 67 of the PCM.

Circuit A142 splices to supply voltage to the upstream and downstream heated oxygen sensors.

The A142 circuit also splices to cavity 6 of the PCM connector. The input by circuit A142 tells the PCM that the ASD relay is energized.

Circuit K41 (K141 2.5L) delivers the signal from the upstream heated oxygen sensor to the PCM. Circuit K41 (K141) connects to cavity 30 of the PCM connector.

Circuit K141 connects to cavity 29 of the PCM connector.

The PCM provides a ground for the upstream and downstream heated oxygen sensor signals (circuits K41 and K141) through circuit K4. Circuit K4 connects to cavity 43 of the PCM connector.

Circuit Z1 provides the ground path for the heater circuits in the sensors. This ground terminates at the right frame rail.

NAME OF CODE: O2S Stays Above Center (Rich)

When Monitored:
With the engine running in closed loop and the coolant temperature above 170°F.

Set Condition:
The oxygen sensor signal voltage stays between 0.5 and 1.2 volts without toggling for at least 8 minutes.

Possible Causes:
- High fuel pressure
- Other engine sensor calibrations
- Ignition system failure
- O2 sensor failure
- Mechanical engine problem

NAME OF CODE: O2S Stays Below Center (Lean)

When Monitored:
With the engine running in closed loop and the coolant temperature above 170°F.

Set Condition:
The oxygen sensor signal voltage stays below 0.5 without toggling for at least 8 minutes.

Possible Causes:
- Low fuel pressure
- Vacuum Leak
- Other engine sensor calibrations
- Ignition system failure
- O2 sensor failure
- Mechanical engine problem

NAME OF CODE: Upstream O2S Shorted To Voltage

When Monitored:
With the ignition key ON.

Set Condition:
The upstream oxygen sensor signal voltage is above 1.2 volts for 3 seconds.

Possible Causes:
- Sensor output wire shorted to another circuit
- Dirty/wet connection causing voltage tracking in connector
- O2 sensor failure
- PCM failure
- Connector terminals
- Connector wires

NAME OF CODE: Upstream O2S Stays at Center

When Monitored:
With the engine running for more than 2 minutes and engine temperature greater than 176° F.

Set Condition:
The upstream oxygen sensor signal voltage is 0.5 volts for 1 minute.

Possible Causes:
- Sensor output wire open
- O2 sensor failure
- PCM failure
- Connector terminals
- Connector wires