Chapter 34 Advanced Diagnostics 673 Copyright by Goodheart-Willcox Co., Inc. Figure 34-25. An oscilloscope can also be used to check the output signal from an oxygen sensor when trouble is hard to find. A—Ground one lead and probe through the connector using jumper wires. Do not pierce the oxygen sensor wiring. B—The oxygen sensor’s signal output should vary as shown. Also note the millivolt readout on the scope face. C—This is a typical waveform for a zirconia O 2 sensor. (Fluke) Ground Scope Probe Through Wire Using Jumper Wires O2 Sensor A B Maximum Peak Voltages Should Reach at Least 800 mV or More Minimum Peak Voltages Should Reach at Least 200 mV or Less Peak-to-Peak Voltages Should Be at Least 600 mV or Greater with an Average of 450 mV C Figure 34-26. With a dual-trace scope, you can compare sig- nals from primary and secondary oxygen sensors in systems with multiple sensors. If the waveform from the secondary O 2 sensor is too similar to the primary sensor, the catalytic con- verter is not functioning and should be replaced. You should also investigate the common causes of converter damage. Primary O2 Sensor Signal Secondary O2 Sensor Signal Figure 34-27. A dual-trace scope can be used to compare oxy- gen sensor and MAP sensor voltage waveforms. Both can be viewed as you shut off a fuel injector. Both waveforms should change when the fuel mixture is leaned out by turning the injec- tor off. 851 328 17 02 Sensor Signal 1.76 1.53 1.41 MAP Sensor Signal Injector Disabled State Figure 34-28. Here waveforms for two oxygen sensors are com- pared with a dual-trace scope. When the throttle is snapped open, both waveforms should change at the same rate and val- ues. (OTC/SPX Corp.) 0.91 0.43 0.30 0.90 420 0.47 Throttle Snapped Open and Closed