# O2 sensor out, check engine light on. How do I pass smog test?

**URL:** <https://community.cartalk.com/t/o2-sensor-out-check-engine-light-on-how-do-i-pass-smog-test/51113>\
**Category:** Maintenance/Repairs\
**Tags:** audi, sensors, a4, engines, lights\
**Created:** [June 30, 2010, 7:51pm UTC](https://community.cartalk.com/t/o2-sensor-out-check-engine-light-on-how-do-i-pass-smog-test/51113 "2010-06-30T19:51:11Z")\
**Posts on this page:** 6\
**Page:** 1

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**Author:** ![tk94127](https://avatars.discourse-cdn.com/v4/letter/t/7cd45c/32.png) [@tk94127](https://community.cartalk.com/u/tk94127)\
**Post date:** [June 30, 2010, 7:51pm UTC](https://community.cartalk.com/t/o2-sensor-out-check-engine-light-on-how-do-i-pass-smog-test/51113/1 "2010-06-30T19:51:11Z")

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I have a 99 Audi A4. Great car. Check engine light comes on, o2 sensor is out. $700 to replace. No other damage. Is there any way to pass the smog test without replacing this?

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**Author:** ![texases](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/texases/32/862_2.png) [@texases](https://community.cartalk.com/u/texases)\
**Post date:** [June 30, 2010, 7:54pm UTC](https://community.cartalk.com/t/o2-sensor-out-check-engine-light-on-how-do-i-pass-smog-test/51113/2 "2010-06-30T19:54:21Z")

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Would kinda defeat the purpose of a ‘smog’ test, now wouldn’t it?

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**Author:** ![tk94127](https://avatars.discourse-cdn.com/v4/letter/t/7cd45c/32.png) [@tk94127](https://community.cartalk.com/u/tk94127)\
**Post date:** [June 30, 2010, 7:58pm UTC](https://community.cartalk.com/t/o2-sensor-out-check-engine-light-on-how-do-i-pass-smog-test/51113/3 "2010-06-30T19:58:22Z")

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I don’t understand. The sensor’s out, the car’s running fine. The check engine light prevents the test from going any further

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**Author:** ![texases](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/texases/32/862_2.png) [@texases](https://community.cartalk.com/u/texases)\
**Post date:** [June 30, 2010, 8:15pm UTC](https://community.cartalk.com/t/o2-sensor-out-check-engine-light-on-how-do-i-pass-smog-test/51113/4 "2010-06-30T20:15:40Z")

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Your check engine light is on because the sensor, which controls the amount of fuel getting to the engine and the performance of the catalytic converter, apparently isn’t working. That’s why having that light on is an automatic ‘fail’. $700 for an O2 sensor is very high. Is this from a dealer? If so, use the mechanic finder under ‘actual car info’ to find an independent mechanic in your area.

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**Author:** ![tk94127](https://avatars.discourse-cdn.com/v4/letter/t/7cd45c/32.png) [@tk94127](https://community.cartalk.com/u/tk94127)\
**Post date:** [June 30, 2010, 8:22pm UTC](https://community.cartalk.com/t/o2-sensor-out-check-engine-light-on-how-do-i-pass-smog-test/51113/5 "2010-06-30T20:22:22Z")

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Ok, I get it now. Thank you.

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**Author:** ![Bladecutter](https://avatars.discourse-cdn.com/v4/letter/b/aeb1de/32.png) [@Bladecutter](https://community.cartalk.com/u/Bladecutter)\
**Post date:** [June 30, 2010, 9:21pm UTC](https://community.cartalk.com/t/o2-sensor-out-check-engine-light-on-how-do-i-pass-smog-test/51113/6 "2010-06-30T21:21:36Z")

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You didn’t mention if it’s the V-6, or one of the Turbo 4 cylinder engines.

Anyway, you can buy the oxygen sensors for ~$100 each online.  
Installing them is easy.

The 4 cylinder engines have 2 sensors, while the 6 cylinder cars have 4.

And no, you can’t pass an emissions test with a CEL on.  
Can’t be done, unless you’re slipping the tester a bit of money.

BC.