hybrid90

My Civic has a cracked manifold and I've been driving it for about 3 months now. I just ordered a new manifold and was wondering if I should also replace the O2 sensor that's after the cat.

I have a new O2 sensor for the exhaust manifold but not for after the cat... is it really necessary to replace that one?

TSI_shinobi

Do you have a code for the down stream sensor?

like P0141?
bank 1 Sensor 2 code? If not I wouldn't worry about it, the cracked manifold really cant hurt the sensor (unless it has like a golf ball sized hole in it where liquids could touch the probe)

some people like to do it as preventative maintenance, when they are doing exhaust work. I don't like to spend money unless i have to

hope that's helpful .

hybrid90

There was a code but don't remember which one. The crack is from the hole of the upstream O2 sensor down towards the second runner so I believe the code is for that sensor only.

Since my exhaust mani is retardedly connected to the cat (thanks Honda for the dumb design).. I'm replacing the whole thing that's why I was wondering if I should replace the downstream O2 sensor as well. Should be ok not to replace downstream O2 right?

motegicivic

What model do you have?

For the secondary o2, no code I would not worry about it, the primary is more important.

hybrid90

98 Civic LX, the one with the 1-piece exhaust manifold/cat combo

beecee18

This is where you are wrong,

You said you have a code, my guess (system to lean).

Any time you introduce an openening (hole/crack) before the O2 sensor which should be sensor 1 bank 1, You are increasing the chance that more oxegen (air from the leak) reads on that sensor.
And that sensor is reading your afr and constantly adjusting, so you introduced air before sensor so the ECU goes ahead and blasts a system to lean code to defend the erronious reading thats being caused by the leak.

If this is the code your car shows congrats, you are probably getting 1/3 of the gas mileage you were cause this code on most applications will over fuel to compensate for the introduction of oxegen from the leak, This happens well after the fuel trims arent enough to combat it. Replace manifold then reset ECU.

My advice if the fix can't be performed quickly, is to keep resetting the ecu when the code comes up, because we know there is no mission critical problem making the code come on. Its just air getting to the sensor when it shouldnt be.

beecee18

P0171 would be the sytem to lean.

If your lucky the leak is either far from sensor thus not affecting it,
Or the leak is not big enough so the fuel trims can combat it.
Which still funks up your gas mileage.