Oxygen Sensor Monitor
BACKGROUND
Effective control of exhaust emissions is achieved by an oxygen feedback system. The most important element in this system is the upstream oxygen (02) sensor mounted in the exhaust manifold.
OPERATION
Once it reaches its operating temperature of 572°F to 662°F, the sensor generates a voltage inversely proportional to the amount of oxygen in the exhaust. This information is used by the Powertrain Control Module (PCM) to calculate the fuel injector pulse width necessary to provide the critical 14.7:1 air/fuel ratio (stoichiometric).
A properly operating sensor must be able to generate an output voltage within its operating range of 0 to 1 volt, as it is exposed to different O2 levels. It also must be able to detect these changes quickly. To detect a shift in the air/fuel mixture (lean or rich) the output voltage must change beyond a threshold value. A malfunctioning sensor could have difficulty changing beyond the threshold value.
While there are existing tests to determine if an O2 sensor is shorted to voltage, contains an open circuit, or does not function at all, prior to OBD II, it was impossible to identify sluggish or marginal sensors.
A slow response rate (see figure above), or reduced voltage output are both signs of an improperly operating oxygen sensor. This could mean longer instances of combustion of less-than-optimum air/fuel mixtures, which can significantly increase emissions.
The 02 sensor must have a voltage output of greater than 0.745 volt in order to pass the test. It must also switch more times during a 120-second period than a predetermined threshold (to demonstrate quick response), or produce a pre-determined number of steep-slope switches (demonstrating dynamic range and quick response) over the same time period. Data is not stored unless idle time exceeds ten seconds.
ENABLING CONDITIONS
The following conditions must be met before the oxygen sensor monitor will run:
^ Engine coolant temperature greater than 170°F
^ Approximately three minutes elapsed time after start-up
^ Vehicle has operated at more than 24 MPH and 170°F for a accumulative total of at least two minutes
^ Vehicle is in drive (automatic transmission only)
^ High-pressure power steering switch is OFF
^ Vehicle is at idle with RPM between 512 and 864
^ This test may be inhibited if the A/C is cycling too rapidly. (Testing with the A/C off is suggested.)
PENDING
The oxygen sensor monitor does not run if the MIL is illuminated due to one of the following:
^ Misfire DTC
^ Upstream O2 heater DTC
^ Vehicle speed sensor DTC
^ The vehicle is in the limp-in mode due to MAP, TPS, or engine temperature DTCs
^ Park/Neutral switch rationality DTC
^ Front O2 sensor electrical DTC
CONFLICT
The oxygen sensor monitor does not run if any of the following are present:
^ Fuel system rich intrusive test
^ Time after start is less than 60 seconds
^ One trip misfire maturing code
^ One trip upstream O2 heater maturing code
SUSPEND
There are no suspend conditions for the oxygen sensor monitor.
The O2 monitor runs once each trip after the enabling conditions have been met. The MIL is illuminated and a DTC stored if the O2 sensor does not produce a 0.745 volt output and/or does not perform enough switches within the test threshold for two consecutive trips. The message on the scan tool screen appears as follows:
Upstream 02S Response
Flash Code - 21 J 2012 Code - P 0133
The lamp extinguishes 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 of the O2 sensor could possibly be caused by problems with any of the following components:
1. Exhaust system (leaks)
a. Pipes
b. Manifold
c. Catalytic converter
d. Seals
2. Wiring (see figure above) and harness and connectors
3. Fuel
4. PCM