# How many Oxygen sensors do I need?



## noquacks (Jun 5, 2010)

Have a 2001 civic ex, 1.7L engine, and wanted to buy a new O2 sensor, but they list an upstream and a downstream. Didnt know I needed 2, or do I? Should I replace both? original is in there now, 140,000 miles. Im orig owner...


Are both easy to get to?

thanks, people


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## noquacks (Jun 5, 2010)

Oh- they keep telling me I ned to change them, as my gas mileage has gone down very noticeably. They also said they can go bad and not trigger an engine code at all- kinda like a dummy light- goes on only when extremely bad........theyre cheap, so Im not so against the idea.


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## nap (Dec 4, 2007)

how many do you need? You need all of them for the car to operate properly. 


If you are replacing them as a guess or as a prophylactic, then replace both of them. Given you have no trouble codes (the "check engine light would illuminate), there is no indication either is bad so if you are doing it because it might help, then do both. 

I wouldn't hold my breath waiting for things to show improvement though. Even if they are nearly of spec in their readings, that should not allow for a major change in efficiency of operation of the engine. That is why they allow a range when checking their output. 

and I have to say; most O2 sensors are intentionally placed in the most inaccessible area on the exhaust system. 


just kidding. No idea where yours are. Some are very easy to get to. Some are extremely difficult to get to but regardless, you want to purchase an O2 sensor socket to install them.


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## Windows on Wash (Aug 30, 2011)

nap said:


> and I have to say; most O2 sensors are intentionally placed in the most inaccessible area on the exhaust system.
> 
> 
> just kidding. No idea where yours are. Some are very easy to get to. Some are extremely difficult to get to but regardless, you want to purchase an O2 sensor socket to install them.


Joking...but often true. Get the O2 socket as well. It will pay for itself a gazillion times over.


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## noquacks (Jun 5, 2010)

good to know- not to expect slam bang results on fuel mileage. man, though, what else could it be? Air filter? PCV valve cleanup (never done)>....


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## nap (Dec 4, 2007)

Have you ever replaced the spark plugs? Plug wires (if applicable)? Distributor cap (if applicable)?


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## noquacks (Jun 5, 2010)

nap said:


> Have you ever replaced the spark plugs? Plug wires (if applicable)? Distributor cap (if applicable)?


yes, new in 2009, about 50,000 miles ago.. No plug wires. No distributor. oops, maybe its time(?)....LOL


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## Eddie13 (Apr 20, 2015)

Upstream O2 is before your catalytic converter, it gives the computer feedback on whats in your exhaust after engine combustion so it can tune its air fuel ratio accordingly, Directly affects gas mileage.. 

Downstream O2 is after your cat and lets the computer know if your cat is working properly or not so it can throw a code if it isnt so the govt can more easily fail you on emissions..


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## Brainbucket (Mar 30, 2015)

Yes, the upstream O2 (in front of cat) is the main one the PCM looks at to make adjustments for engine management. The rear has nothing to do with engine managment but is only there to monitor cat. So if you want to 'change an O2', just change the front one. If the back one is bad, light will come on. 140000 miles I bet it needs a fuel injector/ intake flush. BG is good but I like Motorvac MV3. It's the best imho. The stuff you put in your tank is snake oil and sometimes make problems. Throttle body clean with an old toothbrush and cleaner. Just hold accel pedel down and clean away. If it has a MAS Air Flow sensor, it too could use a clean with electronic contact cleaner. Just spray the crap out of it. Don't touch the inside other than spray. It doesn't take to much. Don't use anything else as it doesn't leave any resadue.


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