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Downstream O2 sensors

Im new to this forum so be kind hahaha.

I have a 2006 fiat stilo 1.6 16v. She has a CEL with a trouble code P0141.

I have symptoms:
Slight decrease in fuel economy
Rough engine idle which started out first eventually followed by…
Slow acceleration
Underperforming engine low power.
Not in limp mode
CAT removed by previous owner.

I read a thread on here from years ago about a downstream o2 sensor.

When the air outside is really cold like early morning winter days then she accelerates better but in normal condition she has a sluggish acceleration.

I replaced spark plugs and fuel pump.
I fitted a K&N cone air filter because of air box falling out and cracking while reversing out my garage. Can’t get a box filter for her in my country anymore.
However she never drove sluggish after fitting the cone. It was some few months after that she started to give acceleration symptoms. Rough idle was present during box filter use already. New Fuel pump made it idle better. Before it would switch off at times .

Thanks in advance

That would set a check engine light and it will not go away. If the downstream O2 sensor was removed as well, it could set this code.

A P0141 is a malfunction of the HEATER in the downstream O2 sensor. The heater is used to warm up the sensor more quickly so it will start working.

You seem to have all the symptoms listed for this failure. I suggest you install a new catalytic convertor and replace the downstream O2 sensor if you want this car to run properly.

Thanks for the reply.

Downstream is still connected but with the trouble code.

I can price a CAT if local part store has. This car is tough to get parts for

Hi Darryn:

CAT removed by previous owner.

If the CAT was removed by the previous owner, was the P0141 CEL always on? Or did it just recently show up?

Hi Joe.

It’s been on for a while. It’s my partners car and he ain’t sure when he bought it but he don’t think it was on lol. But as long as this car been giving these symptoms ,the CEL has been on. It started last year. Times the car drives good and times it really lazy. Before I got a scan tool,I replaced fuel pump,plugs,injector cleaner,air filter changed,crank sensor,timing belt(was due for replace)oil and filter.
On tool it says O2 value downstream is 0v but flashes green light saying it’s connected.
Cleaned both O2 sensors and they were both covered in thick carbon.
O2 1x1 is upstream and O2 1x2 is downstream.
Values are at idle.

Closed loop at idle. When I cleaned them up etc it said closed loop using 2 O2 sensors then jumped back to “using 1 o2 sensor feedback fault”

Some say downstream doesn’t affect performance and others do. Which is correct lol

The downstream O2 sensor does not affect performance. It’s there to make sure your CAT is functioning correctly. If you really have no CAT, then either the light has always been on, or there’s something like a “spark plug non-fouler” in there (illegal) that kept your CEL off.

Given all the driveability issues you reported in your opening post, I don’t believe they’re related to your downstream sensor.

Hi. There is a non fouler on.

I cleaned out my Cone today and engine breather so car drove better.
In the engine breather pipe there was lots of sludge and there is a sort of siff in the pipe that was full of dirt and sludge.
Driving it abit harder to clean her out. The more I’m driving after cleaning,the better she seems to pull.

Could it have been lack of breathing because she also doesn’t rev until red line. Just stops at about 5500rpm. And takes long to get there when driving.

After cleaning my acceleration got better it seems.

The downstream sensor has a heater circuit problem. You cant clean that. As a matter of fact you can’t clean an o2 sensor. The carbon on the outside is normal and does not affect what’s going on inside the sensor. Get a new friggin sensor for $40. A catalytic converter would help too because the computer will react to the values provided by the the downstream sensor, which will always be way out of specs because of the missing catalytic converter.


I would but no parts store here has the downstream.

I need to order online which was my plan for April 2020 but covid made me lose my job. My industry is dead right now.

No CAT could have burn out the downstream?

I doubt it would have caused it to burn out. Given the car is 14 years old, the O2 sensor could be defective.

Another possibility, you said the downstream O2 sensor uses a “non-fouler” to keep the O2 sensor happy. Oftentimes that non-fouler needs to be packed with a little wad of steel wool to buffer the air flow past the sensor. If that’s the case with your car, see if repacking it helps.

No, it would not. It wouldn’t be uncommon for an O2 heater element to burn out. It did its job for 14 years and constantly heated the sensor while the engine was running.

Unless you live in a place/country that does not require emission testing I am surprised the car passed testing. The lack of a catalytic converter is hard to hide when they roll that mirror thingy under the car to look for it.

I stay in South africa.

No emissions testing here. Maybe during roadworthy but not sure.

Could Lack of CAT and faulty sensor be giving ECU wrong readings causing it to give driveability issues?

Previous owner replaced sensor already I saw. It was replaced with a sensor without connector. (The ones with bare end wires) thats how i know it was replaced.

I cleaned out filter and engine breather pipe and throttle body. She ran better and isn’t as sluggish as before. She pulls abit better. Hesitation in acceleration isn’t as noticeable but still feel a lack of power.

I dont know if you saw the screenshot I posted about my readings.

Thank you for the advice.
Im scared at throwing money at the car which I did already and it wasn’t that.
I will gladly replace sensor but some say downstream does and some say it doesn’t affect performance. Im scared im going to replace it and still same issue(from fiat its pretty pricey and online its cheaper but shipping is expensive to south africa)

I dont know if i mentioned,I got a white ish grayish smoke on sudden acceleration with fuel smell but only on sudden acceleration. After cleaning filter etc I don’t notice smoke.

Yes, of course it can and will. Depending on the reading the computer adjusts the air/fuel ratio making the engine run too lean or too rich. This can make your car stutter, especially at take off or exhaust unburned fuel out the exhaust. Neither one is good and give you an error code.

There are ways to work around this, but it’s mostly illegal in the US.

There is not enough information in your screenshot. What is shown is within specs. But, it’s the reading of only one O2 sensor.

If parts are hard to get for you, my suggestion is to replace the bad sensor and start from there.

Thanks for the reply.

My downstream sensor has no reading at all. No voltage at all. Just a green flashing light(think it’s shown in the screenshot)showing its connected but not picking up.

I’m also thinking replace that first. I read that in more modern cars,the downstream fine tunes AFR according to what the CAT burnt up.

Am I correct ?

In your case, the computer gets the values from both sensors, compares the values and makes the necessary AFR based on what the downstream sensor measured (voltage sent to the computer).

Hi Kurtwm1:
Are you sure of this?

In my experience, a defective downstream O2 sensor will trip a CEL because it’s checking for CAT efficiency. But it won’t cause driveability problems. That’s for the upstream sensors.

It’s also not my experience that a missing CAT could damage a downstream sensor.
A missing CAT will cause a CEL to turn on, unless a workaround like a spark-plug-non-fouler has been added. It sounds like that’s what he has in there.

Feel free to enlighten me.

From the article linked above:

A New Role for O2 Sensors with OBDII

Starting with a few vehicles in 1994 and 1995, and all 1996 and newer vehicles, the number of oxygen sensors per engine has doubled. A second oxygen sensor is now used downstream of the catalytic converter to monitor the converter’s operating efficiency.

Hi Kurtwm1:
I read the article and it too indicates that the downstream O2 sensor is used to monitor CAT efficiency.

I didn’t want to confuse Darryn and have him believe that a defective downstream sensor would cause him driveability problems.

That’s what I said… :…based on what the downstream sensor measured…"