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  OBD2 - Faulty HO2S

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OBD2 - Faulty HO2S
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OBD2 Faults

Faulty Upstream HO2S



Andy, a 24V owner, purchased an OBD Lead and checked for DTCs. The car runs fine, and to his surprise he found three DTCs and put them up on the Forum.


Having recently purchased an OBD lead and having a play around with it, I've found the same faults showing all the time, namely P1131, P1132 and PO132. According to the info this is showing a faulty HO2 sensor (although I haven't checked the wiring to it yet). Anyone enlighten me on this? is it easy to change? does it need changing? is it expensive? (it's the pre cat one). Is it only accessible from underneath??

Any ideas?

There was some discussion amongst several of the members of the Forum in which it emerged that Andy had cleared the codes once, but that they had all returned after a few days. He had found the codes and understood the principle shown in the OBD pages here, but didn't know where to go next.


I posted a suggestion - no, a polite one :-)




You have collected three DTCs which are explained on the HO2S Monitor page and if you check the page you'll see that they all relate to the same sensor - HO2S11, which is the Upstream lamda sensor on Bank 1, (drivers side).

You have cleared the codes in case they are old from before the CAT was changed (they would clear naturally after 3 clear runs anyway) - but if a DTC returns it needs investigating. 

P1131 means that the 11 sensor was indicating well into the Lean for too long - in other words, the bank was running far too lean, or the sensor is duff (because a failed sensor can produce P1131 also.)
P1132 means exactly the opposite - the bank was running too rich !!

P0132 is a generic OBD code that shows the 11 sensor with too high a voltage - this further indicates that the sensor may be duff - or it could be a problem with the wiring.

We really need to watch this sensor. With the engine running, connect with the OBD, then go to DATA page, and select:-
HO2S11, 12 (and 21 and 22 if you have the 24V) and

LTFT1 (and LTFT2 if a 24V)
and then take a drive round the block with the scan running. (Make sure the reconnect box is ticked on the Scan page) NOTE: Don't just click on PID All and read everything - the refresh rate is far too low.
Then STOP the scan and when prompted name the file allante01.

Then zip the three scan files together (.dat .txt and .log) and mail them to me and I'll have a look.


Andy posted his scan sharpish. I unzipped the files and ran the Vehicle Explorer software.  Bingo!



Within a few seconds of the scan starting, this is a plot of the HO2S sensors. The red trace shooting off the page is the HO2S11 sensor, (actually at 1.274 volts) and this does not return to proper range until five or six seconds later. There you go, P0132 - HO2S11 sensor Out of Range - the lamda is supposed to provide a voltage of between 0.3 and 0.9 volts max.


But wait, what else is happening? Yes - lack of switch. Look at the khaki plot beneath. This is the other upstream Lamda sensor on Bank 2 of Andy's 24V and this shows a reasonable 'switching'. This is normal and proper, and the HO2S Monitor is constantly checking the HO2S sensors for this switching. If it detects more than a few seconds of no switching at all it stores a DTC - in this case, high voltage for at least eight seconds. A High voltage means fuel system on this Bank is rich - hence P1132 - HO2S lack of switch indicating Rich


We cannot be sure that this was the actual incident that generated the code, but it probably was, or another just like it.


Now I let the data run through the GRAPH for a few more seconds, and found this:



This illustrates both the P1131 and what the OBD was trying to to about it. Here the red line is still the same HO2S11 sensor. It is 'flatlining' - if this were a person's heart-rate they would be dead. It stumbles along at the merest fraction of a volt and this causes the obverse DTC to P1132 - P1131 - HO2S11 Lack of Switch, sensor indicates Lean. But look what is happening to the blue trace - this is the downstream sensor on the same Bank - HO2S12 - and it climbs steeply, maintains 0.7v for two seconds or so, and then falls away again. This is showing the OBD response to a flatlining sensor - it is deliberately enrichening the mixture to try to 'shock' the HO2S11 sensor into activity. Sometimes this works, but not in this case.


To ensure that this activity is not the result of a blip on the throttle (pressing the accelerator automatically enriches the mixture) I have added the grey trace on the bottom to show RPM. Yes, there is a tiny blip on the throttle but the enrichment was already underway when this blip occurred. and goes on long after it finishes, so it is not connected.


Below, OBD has another go. The HO2S11 is on the operating table in Casualty, OBD is watching it closely for signs of life and tries resuss again - it readies the plates and shouts 'stand clear' and lashes the HO2S11 sensor with a Rich mixture:-



... but nothing. Dead? 'Fraid so, it's a DOA. Call the next of kin ... and post the DTC.  Yes, it's sad, but an advanced chemical reaction like this can't last for ever ...


Now OBD has a problem. It is getting no help at all from the upstream sensor, on which it's fuel strategy depends. This sensor is supposed to indicate to the Fuel Monitor when the system needs to go to Rich, and when to turn to Lean. Oh dear - catalysts and emissions are at risk - and the answer?



Toward the end of the trace, here is switching - of a kind, but it's from the post-cat sensors. It shows that the Fuel Monitor is still going rich-lean-rich in order to force the catalysts to give up its oxygen and stay on the button. I've seen this phenomenon several times before - and you can induce it deliberately by disconnecting a front HO2S sensor. You'd get a DTC reporting the loss - but within a minute or so this switching will start.

The Adaptive Strategy inside the Fuel Monitor appears to be using the Downstream sensor to control fuelling. Clever? Yes, I should cocoa.


And the New Catalyst?


Andy reported that he had been told that one catalyst had been changed on his car. From these plots we can see which catalyst it might have been, and it's not the Bank 2 passenger side. If it were a relatively new catalyst the grey trace on this middle plot would have been a steady straight line showing that the catalyst was at peak efficiency, and Bank 2 is certainly not a straight line - so if a Catalyst was replaced it had to have been on Bank 1 - the drivers side. We can't see proper evidence of that yet because the upstream sensor is duff on Bank 1, but if this is so it may be that the HO2S sensors suffered some damage at the same time as the catalyst.


One more check is needed - the Long Term Fuel Trims - LTFT1 &2. Why? Well, because they indicate if there is a problem with the fuel system. An HO2S sensor can fail to switch for two reasons - one, that it is faulty - but also because the fuel system itself is not switching between rich and lean. The sensor may not not failing to switch - it could be that there is no change to report and it maintains the same voltage.


If the fuel itself is not switching, the LTFT will tell us so, because the Long Term fuel trim will be reduced or increased steadily in order to force the fuelling system to change, so these are the tell-tales. In Andy's case the LTFTs were quite normal.


Conclusion? Check wiring for damage or chafing, connector for corrosion. Change HO2SII sensor.


Andy did so and a few days later replied to me on the Forum:


Just as a bit of feedback from my own probs which you kindly sorted. Replacing the pre cat HO2 sensor has resulted in a lot smoother engine whilst warming up and the average mpg, which I didn't reset, is slowly climbing up. Was stuck around 19 for weeks but is now approaching 22.


And that's what this site is all about. Owners fix their own problems without paying for expensive main dealer time.


Thanks to Andy for the use of his Scan.







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