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Discussion Starter #1
Just had a check engine light stay on for the past couple of trips on my '03 Aero. Read the code as P1141, which as I understand it, refers to the rear/downstream (post-cat) oxygen sensor. The car drives fine presently, and accelerates as expected. Is replacing the rear oxygen sensor likely a simple fix for this code?

I'll be able to order the part in and work on the car some time within the next month. In the meantime, will not replacing the rear oxygen sensor for a long period of time cause any issues for me driving the car (in terms of damage, excess fuel usage, fouling spark plugs etc.)? Or is this more an emissions thing?
 

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Hi AeroChris,

Mimmi's "maybe" response quite correct - you need some more data to identify the issue .

I suggest that you get yourself a logging OBDII tool and log data for your O2 sensors, front and rear. I have not needed to do this (yet) on the two 9-5's I currently maintain, but I have had to on my 2005 BMW.

The two Apps I am most familiar with are Torque and OBD Fusion. I have ODB Fusion and I use it with an iPhone and a VeePeak bluetooth OBDII addapter. You can set up to log specific PID's (such as O2 sensor 1 and O2 sensor 2), start from cold and idle it until warm (4 minutes minimum), and see how the sensor voltages change.

The cool thing about the data logs is that you can email yourself a CSV file with the logged data and look at your sensor response(s) on your laptop, tablet, or phone - you can plot them or whatever you like. Sensor 1 data are used to control air and fuel - Sensor 2 data are USUALLY only to tell whether the catalytic converter is working properly. Running with a bad sensor 2 probably won't hurt much - but I would not because any CEL that is constantly on means I might miss the critical CEL error that I won't see because the light is already on. The OE Bosch sensor can be had for about $62 to $80 USD.

Is the 1141 code intermittent ? If it is, AND if you have data logs that suggest the catalytic converter is 'lazy' but the sensor is good, then try a bottle of Techron Concentrate ($4.79 at my Walmart) in your next full tank of gasoline. With a borderline cat, clean injectors might be the thin margin that avoids the CEL.

Hope this helps you
 

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Those logging apps are great. That's how I diagnosed the rear O2 sensor issue on my NG900. I used a cheap Bluetooth OBDII dongle and the Torque app on my phone. I had it display a voltage graph, and it was pretty obvious that the sensor wasn't doing much of anything. The line was way too close to flat. Once I replaced it, the graph showed the correct voltage swings.
 

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Discussion Starter #5
Hi AeroChris,

Mimmi's "maybe" response quite correct - you need some more data to identify the issue .

I suggest that you get yourself a logging OBDII tool and log data for your O2 sensors, front and rear. I have not needed to do this (yet) on the two 9-5's I currently maintain, but I have had to on my 2005 BMW.

The two Apps I am most familiar with are Torque and OBD Fusion. I have ODB Fusion and I use it with an iPhone and a VeePeak bluetooth OBDII addapter. You can set up to log specific PID's (such as O2 sensor 1 and O2 sensor 2), start from cold and idle it until warm (4 minutes minimum), and see how the sensor voltages change.

The cool thing about the data logs is that you can email yourself a CSV file with the logged data and look at your sensor response(s) on your laptop, tablet, or phone - you can plot them or whatever you like. Sensor 1 data are used to control air and fuel - Sensor 2 data are USUALLY only to tell whether the catalytic converter is working properly. Running with a bad sensor 2 probably won't hurt much - but I would not because any CEL that is constantly on means I might miss the critical CEL error that I won't see because the light is already on. The OE Bosch sensor can be had for about $62 to $80 USD.

Is the 1141 code intermittent ? If it is, AND if you have data logs that suggest the catalytic converter is 'lazy' but the sensor is good, then try a bottle of Techron Concentrate ($4.79 at my Walmart) in your next full tank of gasoline. With a borderline cat, clean injectors might be the thin margin that avoids the CEL.

Hope this helps you
So I have an update all.

Funnily enough, I cleared the code, it was fine for a while, but then the code came back. Wouldn't connect to my OBD-II dongle, so I ordered a new one (which finally arrived) to use with the paid Torque app.

Logged both Sensor 1 Bank 1 (no code) and Sensor 1 Bank 2 (corresponds to p1141 code). Sensor 1 Bank 1 showed wide voltage variances, but Sensor 1 Bank 2 stayed consistently at 0.445-0.46, even at the 4 minute idling mark.

I'll test it again from cold later and see if I can work out how to save and upload a graph, but does anyone know if 0.445-0.46 an indication of a faulty Sensor 1 Bank 2 (rear O2) sensor?
 

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If the sensor 2 never moves for sure there is a fault. Simple test is to operate the car in fuel cut off: drive at moderate speed with warm engine (sensors need some time of 30-60s to heat up after start) and release gas pedal. front sensor should go to approx. 0V, and after a small time sensor 2 should follow. If it remains constant, there is a problem.
It could be the crimp or sensor is broken.
NEVER use contact spray on the connectors, it will destroy the sensor.
 

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Discussion Starter #8
Thanks for the suggestions guys. Okay, so before I go and take a look at the wires (it is pouring down here!) I am hoping my two O2 sensor graphs can shed some light on the issue and maybe even determine if the downstream O2 sensor is actually faulty. I couldn't work out how to export the graphs beyond a screenshot, so I've recorded my screen. See link below:


The red line corresponds to Oxygen Sensor 1 Bank 1 (upstream) and the green line corresponds to Oxygen Sensor 1 Bank 2 (downstream, and as per the p1141 code, the potentially faulty one).

My trip was as follows: I let the car idle for about 1 minute, hit 100kph (60mph) for about 6 minutes mostly cruising at constant speed, slowed down, looped back around and cruised at a mostly constant 80kph (50mph), slowing down and returning home, finally letting it idle for another 1 minute before shutting it off.

Based on these graphs, can I rule out both O2 sensors? Am I looking at the cat or something else?
 

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As I said:
Simple test is to operate the car in fuel cut off: drive at moderate speed with warm engine (sensors need some time of 30-60s to heat up after start) and release gas pedal. front sensor should go to approx. 0V, and after a small time sensor 2 should follow. If it remains constant, there is a problem.
Oh, now I read about P1141: current for heating is too low. I was confused by the hint about J16 fix, which refers to the sensor element.
Now your lines maybe make sense. When the sensor is too cold, it could not react and is nearly frozen at the default voltage of 0,45V. This happens especially at idle and low load. (That is the idea of heated sensors, they react faster, independent of exhaust temperature and do not take that long to heat up from exhaust).

Measure resistance of pin 1 and 2, most probably they are not the gold plated ones, should be 9 Ohms.
If this is ok, then there might be a fault in the wiring with J20 joint.
 

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Discussion Starter #10
As I said:


Oh, now I read about P1141: current for heating is too low. I was confused by the hint about J16 fix, which refers to the sensor element.
Now your lines maybe make sense. When the sensor is too cold, it could not react and is nearly frozen at the default voltage of 0,45V. This happens especially at idle and low load. (That is the idea of heated sensors, they react faster, independent of exhaust temperature and do not take that long to heat up from exhaust).

Measure resistance of pin 1 and 2, most probably they are not the gold plated ones, should be 9 Ohms.
If this is ok, then there might be a fault in the wiring with J20 joint.
So based on the graphs, it appears that the rear O2 sensor (or wiring) is faulty and throwing the P1141 code? Rather than an issue with the cat?
 

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Discussion Starter #12
The strange thing is I expected some loss in power, issues idling, accelerating or with fuel economy etc. to accompany the fault code, whether it be an issue with the 02 sensor or cat, but there's been none of that. Everything is running as per normal. So maybe it is borderline. I'll consider putting some injector cleaner/fuel system cleaner through it, and I'll look at replacing the 02 sensor and see how it goes.
 

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Injector cleaner will not help at all against P1141, there is simply no correlation.
P1141 will not create: loss in power, issues idling, accelerating, there is no correlation.
It might have a tiny theoretical impact on fuel economy, but nothing you will notice.

Measure resistance of pin 1 and 2, most probably they are not the gold plated ones, should be 9 Ohms.
If this is ok, then there might be a fault in the wiring with J20 joint.
 

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The rear oxygen sensor is really only there to measure the health of the catalytic converter. Like others mentioned, having a faulty one shouldn't affect performance much at all, if at all.
 

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Discussion Starter #15
Thanks guys. I've bought the sensor and I'll report back how I go. Is this something I can get to with a wrench? Or is there a special tool that is recommended for the downstream location?

I can see there are a variety of different types of oxygen sensor sockets/wrenches.
 

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Discussion Starter #17
If the sensor 2 never moves for sure there is a fault. Simple test is to operate the car in fuel cut off: drive at moderate speed with warm engine (sensors need some time of 30-60s to heat up after start) and release gas pedal. front sensor should go to approx. 0V, and after a small time sensor 2 should follow. If it remains constant, there is a problem.
It could be the crimp or sensor is broken.
NEVER use contact spray on the connectors, it will destroy the sensor.
I replaced the O2 sensor. It was actually easy undoing it with an adjustable wrench and a couple of taps with a mallet...but getting it in was a huge pain.

Working from underneath I struggled trying to tighten it and getting the correct leverage - I started to strip the bolt with my adjustable wrench. Took a look at my built-for-purpose oxygen sensor kit, and that was also a laugh. Struggled through a couple of sockets and managed to get one on but still couldn't access it properly, so I put an extension into the socket from the bottom...which kept falling out. Couldn't tighten it very well either. Eventually made it stay and attached my huge 80cm (31 inch) or so breaker bar and tightened as hard as I could. Even then, I could barely get the crush washer to become seated, but did the best I could. With all the creaking I thought I was going to twist the exhaust with the amount of leverage I had!

The old O2 sensor was fraying and looked worse for wear.

The check engine light was still on no matter how tight I got the sensor, so I cleared it, went for a short drive and it hasn't come back yet. What I did notice, however, was the replaced sensor 2 now has a hugely wide voltage range and rises and falls with the first O2 sensor, which wasn't the case before, just as you said it should. So fingers crossed all is well and the code won't be back. I'll update if it does.

So my advice for others who might be interested in tackling the job: inspect the area and get (or fabricate) the right tool in advance.

Thanks again :)
 

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when the second O2 sensor moves in the nearly exact same way as the first one, most probably sooner or later P0420, broken catalyst, will come up.
if you had big problems installing the new one, most probably the thread in the exhaust was/is damaged. Normally you can put them in with your fingers and only need a wrench for the last 1/4 or 1/2 turn.
 
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