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O2 Sensors: Pre- and Post-Cat Voltage Readings


September 29, 2008
Reaction score
South MS
City, State
Hattiesburg, MS
Year, Model & Trim Level
00 XLT 3.0L
I have a 2000 Ford Ranger 3.0 Flex Fuel (V) with 134K miles that has started running rough and thrown a few ODBII codes. Although the codes are O2 sensor related, I have been told by other forums members in the past that the codes alone don't tell the whole story, so I got a hold of an OBDII scanner with live data streams.

Would any of you guys know what the expected voltage readings on the pre-cat and post-cat sensors should be? This is what I noticed, and unfortunately I didn't note the voltage scale. Numbers were between 0 and 1:

Engine cold and at idle, the voltage on all sensors was all over the place.

Engine warm at idle: voltage on pre-cat sensors dropped down to near zero (< 0.02) and post cat voltage all over the place.