# Faulty Upstream Sensor Cause Error CEL? **URL:** \ **Category:** Maintenance/Repairs\ **Created:** [May 31, 2011, 4:52am UTC](https://community.cartalk.com/t/faulty-upstream-sensor-cause-error-cel/65094 "2011-05-31T04:52:09Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![Rus](https://avatars.discourse-cdn.com/v4/letter/r/51bf81/32.png) [@Rus](https://community.cartalk.com/u/Rus)\ **Post date:** [May 31, 2011, 4:52am UTC](https://community.cartalk.com/t/faulty-upstream-sensor-cause-error-cel/65094/1 "2011-05-31T04:52:09Z")
If the upstream pre-cat sensor is bad could it cause a P0421 CEL error? ---
**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:** [May 31, 2011, 2:59pm UTC](https://community.cartalk.com/t/faulty-upstream-sensor-cause-error-cel/65094/2 "2011-05-31T14:59:40Z")
Not very likely…P0421 is pointing to a problem with the downstream oxygen sensor or the CAT itself… Year, make, model, mileage is always helpful if you want a meaningful answer… ---
**Author:** ![Rus](https://avatars.discourse-cdn.com/v4/letter/r/51bf81/32.png) [@Rus](https://community.cartalk.com/u/Rus)\ **Post date:** [June 1, 2011, 2:15am UTC](https://community.cartalk.com/t/faulty-upstream-sensor-cause-error-cel/65094/3 "2011-06-01T02:15:00Z")
2001 Mazda Protege L4 2L 116,800 ---
**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:** [June 1, 2011, 4:42pm UTC](https://community.cartalk.com/t/faulty-upstream-sensor-cause-error-cel/65094/4 "2011-06-01T16:42:58Z")
You can simply change the rear sensor and hope that cures it or have a shop with a scan-tool monitor the function of the sensor to verify it’s failure. Oxygen sensors have a design life of 100K miles. They cost $65-$85 dollars. Some are easier to change than others… ---
**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:** [June 1, 2011, 5:31pm UTC](https://community.cartalk.com/t/faulty-upstream-sensor-cause-error-cel/65094/5 "2011-06-01T17:31:46Z")
Look at the signals coming from the O2 sensors with a graphing scan tool or a scope. Do a little research on how to interpret them. That will reduce the guesswork. ---
**Author:** ![Rus](https://avatars.discourse-cdn.com/v4/letter/r/51bf81/32.png) [@Rus](https://community.cartalk.com/u/Rus)\ **Post date:** [July 21, 2011, 3:54am UTC](https://community.cartalk.com/t/faulty-upstream-sensor-cause-error-cel/65094/6 "2011-07-21T03:54:50Z")
My Mazda passed smog with one “Not Ready”. Happy, happy!!!