Contour Enthusiasts Group Archives Forums General Troubleshooting O2 sensor placement is 1 before or after CAT?

HI,
I am working on my 97 Contour Ztech, I have a scan tool thing and am getting a 171. Supposedly a bad O2 sensor. SO I check them and it says sensor 2 is giving no response. Well, logic would say I guess that 2 is after the CAT. The other sensor is reading the way it should, so I would just as well return whichever sensor I do not need to replace (bought both before store closed tonight since I need to have this thing good for work tomorrow) if anyone knows which sensor the scanner thinks is 2 I would greatly apreciate a response.
Thank you

I assume the code discussed below is the same as yours. After reading this I wouldn't jump to any O2 sensor conclusions right away.

This was posted earlier by PA 3L SVT:

According to my Service CD, the following list can be causes of code P0171. Once these have been checked, the next test is to see if there is something wrong with the MAF. Might want to start at the MAF first, since cleaning it will be quicker than checking all these. HEGO (or EGO or O2 sensor, all the same thing) isn't even listed here. Don't buy one until you know you need it. If the dealership wants to replace it, ask if they have tested the MAF first. If no, then they are blindly throwing parts at the problem, on your dime. Next thing to ask is why they aren't following the Service Manual troubleshooting instructions. Point them to Diagnostic Routine H41 - DTCs P0172, P0171, P0174, and P0175: Fuel System at the Corrected Fuel Trim. Good luck - this list is long!

quote:
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Possible causes:

Fuel system

-- Excessive fuel pressure.

-- Leaking fuel injector(s).

-- Leaking fuel pressure regulator.

-- Low fuel pressure or running out of fuel.

-- Contaminated fuel injector(s).

-- Vapor management valve (VMV) leaking.

Induction system

-- Air leaks after the MAF.

-- Vacuum leaks (vacuum lines and gaskets).

-- Restricted air inlet.

-- PCV system.

-- Fuel purge system.

-- Improperly seated engine oil dipstick.

EGR System

-- leaking gasket.

-- Stuck open EGR valve.

-- Leaking diaphragm.

Base engine

-- Oil overfill.

-- Cam timing.

-- Cylinder compression.

-- Exhaust leaks before or near the HO2S(s).

l Check intake air system for leaks, obstructions and damage.

l Check air cleaner element, air cleaner housing for blockage.

l Verify integrity of the PCV system.

l Check for vacuum leaks.

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Plus,

You can simply unplug the sensor you suspect, and see if you get a code for that sensor.

Be careful that you are not confusing bank 2 with sensor 2. The Zetec will not have a bank 2.

And yes, many other things can trigger a P0171. My Zetec had this code and it was a dirty MAF.

TB

Thank you for your replies. I am just getting in there and doing the unplug of my suspected one.
Did the MAF check, the MAF is all good as is the rest of the air intake, I drive very little, have had the car since it was almost new. I change air and oil filter regularly. It is good.
I replaced the fuel filter as well. I have the computer thing, and when I cue O2 sensor #2 whichever that one is, it just sits, I know it IS bad, it is just that I'm not sure if what order they put them in Chilton's seems to be no help either, lol

Thank you for your replies. I am just getting in there and doing the unplug of my suspected one.
Did the MAF check, the MAF is all good as is the rest of the air intake, I drive very little, have had the car since it was almost new. I change air and oil filter regularly. It is good.
I replaced the fuel filter as well. I have the computer thing, and when I cue O2 sensor #2 whichever that one is, it just sits, I know it IS bad, it is just that I'm not sure if what order they put them in Chilton's seems to be no help either, lol

Are you confusing code interpretation?
A '97 would give a P0171 System too lean code.
This shouild not be confused with 3 digit codes of earlier vehicles where a 171 indicates O2 sensor.

p.s. Your 2.0L has 2 sensors; #11 front (located on manifold before cat), and #12 rear (located on exhaust after cat).