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I just bought a 2004 Mazda 6 2.3 and to start it you had to have foot on gas and let it idol rough for a couple minutes until it started idoling normal. The check engine light said cam shaft sensor and upstream o2 sensor. I replaced cam shaft sensor and crank shaft sensor and now the car won’t start at all. It cranks and sometimes backfires. Could the o2 sensor cause it not to start? If not what else could it be. Fuel pump turns on.
 

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Do you smell gas when it does start up? Any misfire codes?

Bad o2 sensor in itself typically means the engine has an issue with a rich fuel mixture (bad fuel injectors) or excessive oil consumption which is ruining the o2 sensor. I'd recommend replacing the o2 sensor as the next step and then scan for codes one more time.
 

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I do smell gas when it starts, and today I replaced spark plugs and checked and have spark so that is okay. The back bumper is black from when it did run by the exhaust so idk if that means anything? Would the o2 sensor cause it not to start?
 

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I do smell gas when it starts, and today I replaced spark plugs and checked and have spark so that is okay. The back bumper is black from when it did run by the exhaust so idk if that means anything? Would the o2 sensor cause it not to start?
Black bumper, near the exhaust tip? Not a good sign. Means the previous owner likely drove the car hard. Sounds like you need to replace the injectors to fix the rough/no start issue. I'd also suggest running a good fuel system cleaner such as redline.

The o2 sensor will not cause no start, but it needs to be replaced.
 
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