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Discussion Starter · #1 ·
I have a 17 focus st with 13k on the engine I changed an o2 sensor cause it was giving a reading saying it was reading slowly so I replaced it. But now the car doesn’t really struggle to take off but really struggles to accelerate and build boost. What did I do wrong or what haven’t I done that needs to be done haven’t reset the ecu just replaced the sensor and started it and it’s had this issue since.
 

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I have a 17 focus st with 13k on the engine I changed an o2 sensor cause it was giving a reading saying it was reading slowly so I replaced it. But now the car doesn’t really struggle to take off but really struggles to accelerate and build boost. What did I do wrong or what haven’t I done that needs to be done haven’t reset the ecu just replaced the sensor and started it and it’s had this issue since.
  • What are your mods?
  • Are you tuned?
Which O2 did you replace, the rear? An O2 sensor in a "not ready" state usually has nothing to do with the sensor, but rather the actual tune.

The sensors and wires are quite fragile and can be easily damaged if not careful, when replacing or working on them.

If you're tuned, what tune are you running?
 

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Discussion Starter · #3 ·
  • What are your mods?
  • Are you tuned?
Which O2 did you replace, the rear? An O2 sensor in a "not ready" state usually has nothing to do with the sensor, but rather the actual tune.

The sensors and wires are quite fragile and can be easily damaged if not careful, when replacing or working on them.

If you're tuned, what tune are you running?
I replaced the lower. Car is completely stock and no tune.
 

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What codes is it throwing now?
 

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The lower O2 sensor? Narrow band, Bank 1, Sensor 2? Are you running a factory downpipe or aftermarket catted unit?
 

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Really big difference between the front & rear O2 sensors slow to respond. If the catalytic converter is clogged or damaged it could get ugly, quick.
 
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Really big difference between the front & rear O2 sensors slow to respond. If the catalytic converter is clogged or damaged it could get ugly, quick.
I'm still interested in the events that caused him to look to find that the O2 sensor wasn't in a ready state. People don't just go looking for it, unless it was because he failed inspection.
 

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Discussion Starter · #12 ·
It threw a code originally saying it wasn’t in a ready state once so I cleared it and it came back so I said okay I’ll replace it. When I took it out it was blacker than black.
 

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It threw a code originally saying it wasn’t in a ready state once so I cleared it and it came back so I said okay I’ll replace it. When I took it out it was blacker than black.
We really need to know the exact codes that were being thrown to understand more. The rear O2 is strictly emissions monitoring and would have no bearing on performance.

It exists to check CAT efficiency, and throws a CEL if it detects an emissions problem. Your upstream O2 sensor is an entirely different story.
 

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Discussion Starter · #14 ·
So not sure what changed I let the car sit for 2 days didn’t touch a thing came back from work and it fired right up no problem. I have no clue maybe it needed time maybe a god came down a blessed it it blows my mind but the car is running perfectly and the CEL cleared itself as well.
 

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That makes things worse. Issues like these don't repair themselves.
 

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Discussion Starter · #16 ·
Yeah I’m not sure car only has 13 k on it almost brand new and I’m not new to changing an o2 sensor but it was really weird I rechecked all the electrical connections whole intake system everything looks normal car running as should no issues anywhere so if I do have a problem occur I’ll be sure to post again but so far running since Monday morning and haven’t had a problem yet.
 
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