O2 Heater Monitor

BACKGROUND
For the O2 sensor's to function properly, they must be heated to approximately 572°F - 662°F. To assist the O2 sensor's in achieving this temperature, they are equipped with Positive Thermal Coefficient (PTC) heater elements. Both the upstream and downstream O2 sensors heater elements are fed battery voltage whenever the ASD relay is energized. The ground for the heater elements is routed through the O2 sensors' 4-pin connectors to an external ground. As current flows through the heater elements, the temperature of the O2 sensors increase. As the temperature increases, the resistance of the elements increase, causing the current flow to decrease. The O2 sensors' heaters combined with exhaust gas under most normal conditions, maintain the temperature of the sensors to around 1200°.

After the engine has started, the upstream O2 sensor's information is used by the Powertrain Control Module (PCM) to assist in the air/fuel calculation. If the sensor is not up to operating temperatures, the information given by the sensor may be inaccurate, possibly causing an increase in emissions.

Information provided by the downstream O2 sensor is used by the PCM to calculate the efficiency of the catalytic converter. The sensor must be heated to allow it to function normally, otherwise, the catalytic converter test may be invalid.

OPERATION
The resistance of an O2 sensor changes with temperature and age. It's resistance normally is between 100 ohms to 4.5 megohms. When the temperature of a sensor is increased, the resistance of the sensor decreases. Inversely, as a sensor ages, the resistance increases. It is the resistance of the sensor's output circuit that is tested for proper heater operation, not the heater element itself.






The test begins approximately five seconds after the engine has been turned off as long as the ignition voltage is off and battery voltage is greater than 10 volts (the PCM still operates even though the key is in the OFF position and remains in operation for a finite amount of time). Once the timer has timed out, the PCM sends a 5 volt bias to the O2 sensor's output wire once every 1.6 seconds, and keeps it biased each time for 35 milliseconds. During this portion of the test, the PCM monitors the voltage on the output wire of the O2 sensor. As the sensor cools down, its resistance should increase, causing the PCM to register an increase of voltage. The PCM determines if an O2 sensor has cooled enough by detecting an increase of 0.49 - 1.56 volts higher than what the PCM detected at the beginning of the test. The maximum amount of time to perform this portion of the test is 144 seconds.

When the O2 sensor has cooled enough, the PCM energizes the Automatic Shut Down (ASD) relay for the next 48 seconds. With the relay energized, current should flow through the heater element causing an increase in temperature at the sensor. As the heater warms the O2 sensor, resistance of the O2 sensor's output circuit should decrease. As the resistance decreases, voltage at the PCM decreases. While the ASD relay is energized, the PCM pulses the 5-volt biased signal 30 times. Each time the biased voltage is activated, the PCM senses for a voltage drop. The O2 heater monitor test passes if the PCM detects at least 0.157 volt decrease in 15 out of the 30 pulsed signals.

ENABLING CONDITIONS
The following conditions must be met before the O2 sensor heater monitor can run.

^ The engine must have been running for at least 5.1 minutes before the test can be performed
^ The engine cannot be running
^ Battery voltage must be at least 10 volts
^ 5 seconds must pass after the engine has been shut down to dissipate any gasses

The test aborts any time the key is cycled to run.

PENDING
The O2 sensor heater monitor test does not run if the MIL is illuminated due to:

^ Upstream or downstream O2 sensor failure

CONFLICT
There are no conflicting conditions.

SUSPEND
There are no suspend conditions.

The O2 sensor heater monitor is tested once each time the engine is turned off, as long as the enabling conditions have been met. The MIL is illuminated and the DTC is stored if the O2 sensor's circuit does not show the appropriate decrease of voltage during the test for a total of two consecutive tests. Because the key is off during the test, the MIL illuminates and the DTC is stored only at the beginning of the next key cycle. The message on the scan tool appears as follows:

Upstream 02S Heater Failure
Flash Code - 21 J 2012 Code P 0135

Downstream O2S Heater Failure
Flash Code - 21 J 2012 Code P 0141

The MIL is extinguished if the conditions causing it to illuminate are not repeated for three consecutive trips. The DTC is erased from memory if the monitor passes for 40 consecutive warm-up cycles.

A malfunction in the O2 sensor's heater could possibly be caused by problems with any of the following components:

^ Upstream or downstream O2 sensor (heater element)
^ O2 sensor related wiring or connectors (heater circuit)