help

I am not sure what the graphs are showing. The top one may be the engine RPM but the two lower ones??

For the NHW11, the first O2 sensor should oscillate between ON/OFF as it measures the oxygen level in the hot exhaust stream. If it is flat or not moving, the feedback mechanism to adjust fuel trim would fail. The car would run but have crappy fuel efficiency. It is running 'open loop' which means no trim at all except the earlier tables stored in the engine controller. I would expect to find an overly rich mixture and carbon, a bunch, in the tail pipe.

The second O2 sensor should have no useful meaning if the first sensor is failed. The proper operation of the catalytic converter requires exhaust that oscillates between rich and lean.

BTW, this is the NHW11 forum, 2001-03 Prius. You may want to hit the "Report" button and ask the moderators to move your posting to the "Gen II Technical" group. You'll find people with more understanding of your particular model.

GOOD LUCK!
Bob Wilson

 

Can the moderators split the thread and just move the last two posts to a new thread? If not this thread should stay here and maverick should start a new thread in either the "Gen II Technical Discussions" or "Gen II Prius Care, Maintenance and Troubleshooting" forums

 

I assume this is from a 2G Prius. The two lower graphs depict voltage produced by the air/fuel ratio sensor and the oxygen sensor.

The engine should be run at 2,500 RPM for two minutes. After that warmup period, maintain engine speed at 2,500 RPM. The air/fuel ratio sensor should produce a voltage around 3.3V, and continually oscillate up and down slightly from that average value. The oxygen sensor should fluctuate between 1V and 0V while the engine is running at that speed.