# O2 Sensor or catalytic converter?

**URL:** <https://community.cartalk.com/t/o2-sensor-or-catalytic-converter/57466>\
**Category:** Maintenance/Repairs\
**Tags:** catalytic-converters, chrysler, pacifica\
**Created:** [November 11, 2010, 11:51pm UTC](https://community.cartalk.com/t/o2-sensor-or-catalytic-converter/57466 "2010-11-11T23:51:01Z")\
**Posts on this page:** 5\
**Page:** 1

<div class="post-metadata">

**Author:** ![Gearhead\_Rick](https://avatars.discourse-cdn.com/v4/letter/g/94ad74/32.png) [@Gearhead\_Rick](https://community.cartalk.com/u/Gearhead_Rick)\
**Post date:** [November 11, 2010, 11:51pm UTC](https://community.cartalk.com/t/o2-sensor-or-catalytic-converter/57466/1 "2010-11-11T23:51:01Z")

</div>

About a month ago I got the dreaded P0420 (catalytic converter efficiency low) code on my 2005 Chrysler Pacifica. I did some diagnostics and found the downstream O2 sensor was ‘lazy’ (the voltage wasn’t changing with the upstream sensor). I replaced the downstream O2 sensor and now I get a P2097 code (post cat converter fuel mixture rich) and still the new O2 sensor isn’t changing voltage by more than .1 volts. Any suggestions what else it could be?

---

<div class="post-metadata">

**Author:** ![mcparadise](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/mcparadise/32/32_2.png) [@mcparadise](https://community.cartalk.com/u/mcparadise)\
**Post date:** [November 12, 2010, 12:44am UTC](https://community.cartalk.com/t/o2-sensor-or-catalytic-converter/57466/2 "2010-11-12T00:44:21Z")

</div>

Replace the upstream O2 sensor. It’s much less expensive than a new cat.

---

<div class="post-metadata">

**Author:** ![Caddyman](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/caddyman/32/23_2.png) [@Caddyman](https://community.cartalk.com/u/Caddyman)\
**Post date:** [November 12, 2010, 2:23am UTC](https://community.cartalk.com/t/o2-sensor-or-catalytic-converter/57466/3 "2010-11-12T02:23:46Z")

</div>

Both sensors need to be on the same page, not 100K miles apart…

---

<div class="post-metadata">

**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:** [November 12, 2010, 4:39pm UTC](https://community.cartalk.com/t/o2-sensor-or-catalytic-converter/57466/4 "2010-11-12T16:39:51Z")

</div>

“the downstream O2 sensor was ‘lazy’ (the voltage wasn’t changing with the upstream sensor)”

The downstream O2 sensor is supposed to appear “lazy”.  
The functioning cat converter acts as an O2 reservoir. Like a capacitor to those of us into electronics.

---

<div class="post-metadata">

**Author:** ![AL5000](https://avatars.discourse-cdn.com/v4/letter/a/a8b319/32.png) [@AL5000](https://community.cartalk.com/u/AL5000)\
**Post date:** [November 12, 2010, 5:34pm UTC](https://community.cartalk.com/t/o2-sensor-or-catalytic-converter/57466/5 "2010-11-12T17:34:28Z")

</div>

Your emissions system should still be under warranty. At least the cat would be.