# Misfire! O2 sensor read lean or rich?

**URL:** <https://community.cartalk.com/t/misfire-o2-sensor-read-lean-or-rich/85228>\
**Category:** General Discussion\
**Created:** [May 30, 2014, 7:17pm UTC](https://community.cartalk.com/t/misfire-o2-sensor-read-lean-or-rich/85228 "2014-05-30T19:17:36Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![Mechaniker](https://avatars.discourse-cdn.com/v4/letter/m/13edae/32.png) [@Mechaniker](https://community.cartalk.com/u/Mechaniker)\
**Post date:** [May 30, 2014, 7:17pm UTC](https://community.cartalk.com/t/misfire-o2-sensor-read-lean-or-rich/85228/1 "2014-05-30T19:17:36Z")

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Suppose misfire (eg, no spark) in one cylinder.  
Will O2 sensor voltage read lean (≈0 mV) or rich (≈900 mV)?

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**Author:** ![circuitsmith](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/circuitsmith/32/40376_2.png) [@circuitsmith](https://community.cartalk.com/u/circuitsmith)\
**Post date:** [May 30, 2014, 7:31pm UTC](https://community.cartalk.com/t/misfire-o2-sensor-read-lean-or-rich/85228/2 "2014-05-30T19:31:25Z")

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It will read lean because the dead cylinder pumps out air and unburned fuel.  
The O2 sensor doesn’t measure hydrocarbons, only the oxygen in the air.