Oscilloscope Patterns and Waveforms

TESTING THE OXYGEN (O2) SENSOR USING PROPANE:
Three aspects of O2 sensor performance should be checked. If any one of the three performance checks fails, the sensor should be replaced and the new one tested.

The following procedures and specifications are based on Robert Bosch Corporation testing procedures and vehicle manufacturer's OBD II oxygen sensor monitoring specifications for zirconia type oxygen sensors.

1. Connect And Set Up Propane Enrichment Tool. Connect the propane enrichment tool to a large intake vacuum source (PCV or Brake Booster works well).
2. Connect and Setup the Digital Storage Oscilloscope (DSO):
a) Connect the "COM" probe to the O2 ground circuit or engine block (negative battery terminal NOT recommended).
b) Connect the "CH1" probe to the O2 signal wire to the Powertrain Control Module (PCM). Use a wiring diagram to get the color of the wire or PCM pin.
c) Set the voltage scaling to 200 mV/div.
d) Set the time base to 1 s/div. (1 second per division).
e) Select "GND" coupling and position the trace on the second division line from the bottom of the DSO display.
f) After ground position is set, select "DC" input coupling.
g) Select "NORMAL" acquire mode.
h) "TRIGGER" mode is set automatically (Roll Mode).
3. Warm the engine at 2500 RPM for 2 - 3 minutes.
4. Let the engine idle. The following test must be completed within 30 seconds from this point in the procedure to insure accurate amplitude and response time test results.
5. Slowly add propane until the O2 signal goes rich. A properly operating system will try to pull the O2 sensor voltage lean. Keep adding propane slowly until the system loses its ability to compensate for the rich mixture. Then keep increasing propane flow until the engine speed drops a few hundred RPM from running so rich. This step in the procedure, when performed properly, should take about 20-25 seconds.

Good Oxygen Sensor Pattern (Typical):

Bad Oxygen Sensor Pattern (Typical Failure):

6. Quickly pull the propane off the vacuum source, thus creating a huge vacuum leak. Generally, it's okay if the engine stalls -- it won't affect the test. Turn off the propane valve.
7. Wait until the drop in the waveform moves to about the center of the DSO display and press the "RUN/HOLD" button on the DSO control panel. The test is now complete. Now, the waveform can be examined on the DSO display to give the O2 sensor a passing or failing grade.

EXPECT THESE WAVEFORM RESULTS:
If the sensor is good, it should meet or exceed all of these performance specifications: (see Image)

Oxygen Sensor Parameters:

If the sensor being tested fails to meet ONE of the above performance specs, then it should be replaced and the new sensor should be tested.

IMPORTANT TECHNICAL NOTES:
#1 For the sensor to be good, the above specifications must all be met when the procedure is followed exactly. If the engine idles too long before the propane is shut off and the vacuum leak is created (more than 20-25 seconds), then the sensor may cool down too much. This can lower its maximum amplitude and increase its response time, causing a possible false test failure. If the engine doesn't idle long enough before the propane is shut off and the vacuum leak is created, then the sensor's ability to maintain "Feedback Fuel Control Closed Loop" will not be tested properly. Many sensors will perform adequately when very hot (after 2-3 minutes at 2500 RPM), but will fail one or more test parameters after just 5 seconds of idling. That's why it's important for the propane enrichment portion of the test to consume 20 - 25 seconds.

#2 On some vehicles, it may be difficult or impossible to perform the "lean drop" portion (step 5) of the O2 sensor test. An alternate "Snap-Throttle" O2 sensor test may have to be performed.

#3 If there is any doubt about the visual readings from the waveform, or if the exact test results are desired, use the DSO cursors (or read MAX and MIN off the scope screen on some DSO's) to measure the maximum voltage, minimum voltage and response time. Most bad O2 sensors will be obvious from a visual inspection of the waveform, however.

FOR MORE INFORMATION
Description of Automotive Signals
Diagnosis Using A Labscope
Labscope Quick Reference Appendix