# 2003 GMC Terrain CEL

**URL:** <https://community.cartalk.com/t/2003-gmc-terrain-cel/154097>\
**Category:** Maintenance/Repairs\
**Tags:** terrain, gmc\
**Created:** [December 5, 2019, 1:23pm UTC](https://community.cartalk.com/t/2003-gmc-terrain-cel/154097 "2019-12-05T13:23:45Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![CRAIGBILES](https://avatars.discourse-cdn.com/v4/letter/c/aeb1de/32.png) [@CRAIGBILES](https://community.cartalk.com/u/CRAIGBILES)\
**Post date:** [December 5, 2019, 1:23pm UTC](https://community.cartalk.com/t/2003-gmc-terrain-cel/154097/1 "2019-12-05T13:23:46Z")

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engine light is on it’s coming back to catalytic converter.

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<div class="post-metadata">

**Author:** ![Mustangman](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/mustangman/32/44130_2.png) [@Mustangman](https://community.cartalk.com/u/Mustangman)\
**Post date:** [December 5, 2019, 1:56pm UTC](https://community.cartalk.com/t/2003-gmc-terrain-cel/154097/2 "2019-12-05T13:56:55Z")

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> [@CRAIGBILES](#):
>
> engine light is on

The check engine light?

> [@CRAIGBILES](#):
>
> it’s coming back to catalytic converter

No, it is not. Is the code a P0420? If so, there are 6 things listed it could be before a bad cat.

A code P0420 may mean that one or more of the following has happened:

Leaded fuel was used where unleaded was called for (unlikely)  
A damaged or failed oxygen / O2 sensor  
Downstream oxygen sensor (HO2S) wiring damaged or connected improperly  
The engine coolant temperature sensor is not working properly  
Damaged or leaking exhaust manifold / catalytic converter / muffler / exhaust pipe  
Failed or underperforming catalytic converter (likely)  
Retarded spark timing  
The oxygen sensors in front and behind the converter are reporting too similar of readings  
Leaking fuel injector or high fuel pressure  
Cylinder misfire Oil contamination