Catalytic converter and flex-fuel problems can be tricky to diagnose. Inaccurate data from an oxygen sensor or air/fuel sensor can lead a tech down the wrong path and cause needless replacement of good parts. When dealing with catalytic converter fault code(s) and flex-fuel problems, it is necessary to make sure that the air/fuel (A/F) and/or oxygen sensors (O2S) are accurately reporting.

To do this, you will need to graph or at least monitor all A/F and O2 sensors while forcing them rich and then lean. An oscilloscope scope or graphing digital scan tool is best because of its refresh rates. If you cannot monitor more than one, you can test one at a time.

Because you will need to bias even the downstream sensor(s) it is advised to use propane to drive them full rich so that even the downstream O2S reaches its maximum rich output. But first, you need to verify that the computer enters closed-loop fuel control and that there are no exhaust leaks from the exhaust ports down to about 6 inches past the secondary O2S. An easy way to do this is to use a socket that barely fits into the tailpipe and a vise grip to hold it in while you run the engine and listen for a hissing sound along the exhaust.

The test itself consists of adding propane to the intake until fuel trims reach minus twenty percent (-20%) or more and then shutting it off. The PCM will have been adapting for this very rich condition so when you shut the propane off, the switch to a lean condition will be dramatic. If you have no access to propane, a substitute would be to run the engine at a steady 1200 rpm and snap the throttle wide open (WOT) a few times. 

Your scope or scan tool should display a rise in voltage above 800 millivolts and a drop below 150 millivolts in less than 50 milliseconds on the time scale. Not rising high enough, dropping low enough, or switching fast enough would be causes to change the sensor as long as the vehicle’s computer is in fuel control and there are no exhaust leaks. Note that a scan tool may not report data fast enough to see the sudden changes clearly. A scope or graphing multi-meter is always the best choice for this kind of testing.

If you are testing an A/F (wide band) sensor, be sure to use the appropriate scan tool mode and factory measuring method. If you look at the chart showing Global (Generic) OBD and OEM scan tool modes, you can see why I emphasize scan tool mode. Take a look at the difference between global and OEM mode voltage and notice that some A/F sensors are measured by scan tool reported amperage:

Bad data can ruin your day by wasting time and money replacing parts that are not defective. For example, a P0420 code indicating an ineffective catalytic converter can be set if a sensor is giving you incorrect information. But a Cat is repair is far from cheap. It might be worth confirming the device that monitors the Cat is working correctly to avoid that expense. Proving your oxygen and A/F sensors are reporting accurately can optimize a successful repair and save you your valuable time and money.