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??
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
I posted a suggestion - no, a polite one :-)
You have collected three DTCs which are explained on the HO2S
http://www.fordscorpio.co.uk/ho2smonitor.htm and if you check the
http://www.fordscorpio.co.uk/codes.htm page you'll see that they all relate
to the same sensor - HO2S11, which is the Upstream lamda sensor on Bank 1,
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
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!
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.
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
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:
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.
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.
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:-
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
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?
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.
Adaptive Strategy inside the Fuel Monitor appears to be using the Downstream
sensor to control fuelling. Clever? Yes, I should cocoa.
the New Catalyst?
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.
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.
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
Conclusion? Check wiring for damage or chafing, connector for corrosion. Change
so and a few days later replied to me on the Forum:
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.
that's what this site is all about. Owners fix their own problems without paying
for expensive main dealer time.
to Andy for the use of his Scan.