O2 sensor replacement

[MO_GUY]

when i had my codes scanned the mechanic told me i needed to replace my upstream O2 sensor, which one of them is it? he said there is probably one on the intake, then 1 before the cat and one after......which one is the upstream?

 

[B94Sport]

Someone with an SOHC will chime in, but I am guessing that if those are the only O2 sensors you have, then it is the one before the cat.

There is no O2 sensor on the intake, unless he meant the MAS, which should not need to be replaced, just cleaned if anything.

EDIT: I've been looking a bit and it seems like the SOHC has 4 O2 sensors? One before and one after each of the two cats?

 

[AjRagno]

MO_GUY,

Admittedly, my most extensive knowledge is with the F-150s, but I am getting a lot more experience with our Explorer now that we're approaching 60,000 miles and this should still apply.

From another post, you stated that you have DTC P0171 & P0174. These are lean codes for both banks. It would be rare that both the driver and passenger side of the engine would fail at the same time, so I don't think this is the H02 sensors failing. I think that rather they are doing their job and alerting you to another problem.

Running lean means that you are getting too much air and not enough fuel.

• Dirty MAF (Not measuring all of the air passing into the intake causing a lean condition)
• Vacuum leak. Intake manifold gaskets. PCV valve. Loose/cracked hose on intake manifold or air intake tube.
• Dirty fuel filter.

The most common reason for running lean on both banks is a dirty MAF sensor. If this were my truck, I'd first make sure the PCV valve is properly attached, followed by checking all hoses connected to the intake manifold for cracks or loose fittings. Then I'd clean the MAF sensor and clear the computer's memory by disconnecting the battery for a few minutes. There is no harm in checking these things and then resetting the computer. If it is the HO2 sensors, the MIL will come on again after a few warm up cycles.

I wouldn't discount leaking lower intake O-rings either. Mine were replaced last summer under 00M12 and last week I removed the intake and found carbon deposites running from each port, indicating they were leaking yet again.

For future reference: There are 4 HO2 sensors. The upstream HO2 sensors are mounted on the exhaust manifolds. The passenger side is accessible from the engine compartment and the driver's side is most accessible from under the vehicle. You also have 2 HO2 sensors mounted behind that catalytic converters; these cat monitors rarely fail however, so I wouldn't worry about them.

 

[Glacier991]

Great post AjRagno!

Happy Exploring

Chris