O2 sensor fault but still get data!?

Questions specifically about a Petrol powered Citroen C3 (usually engine or fuel related problems).
Forum rules
One question per topic.
Petrol engine related faults, like injectors, error codes, overheating.
You can post more topics if necessary, but only one question per topic.
User avatar
My Name: Ozvtr

Moderator
Posts: 942
Joined: Wed Jul 10, 2019 4:31 am
Model: C3 2002-2005, Original shape model
Year: 2003 (53)
Engine Size: 1.4 (8v)
Fuel Type: Petrol
Mileage: 80000
Trim Level: Other
Gearbox: Automatic PRND
DPF: No
LHD or RHD: RHD
Engine name: TU3 (75 PS)
Location: Brisbane, Australia.
Has thanked: 36 times
Been thanked: 252 times

Post

Um, this is a fork from the thread I started in the general area about the LEXIA. The thread began to become specifically about the engine symptoms, so because of that I have started this thread.
So if the post seems a bit truncated I'm sorry.
Basically the story is I have a TU3JP(KFV) engine that was in a sad state and I have done a lot to it to get the car back on the road. My woes are scattered all over this site, if you care to look. :lol: The car is now back on the road and it has developed one very frustrating fault!

I'll get a screen shot of the LEXIA later but there are 3 faults in the "injection" area (paraphrased) and I don't have the codes but I'll get them:
Upstream oxygen sensor fault.
Upstream oxygen sensor heater fault.
Catalyst degraded fault.
The Catalytic converter fault doesn't worry me, it will be the fact that the difference between the up and down sensors are not what the engine ECU is expecting (because one is not "working"). Both sensor outputs show the converter is working fine. Yes, I am getting sensor output!!
Now the fact that it's the O2 sensor AND the heater circuit makes me suspect its the sensor connector but changing over to a new sensor makes no difference and the connector is clean and free of contaminants and I am getting sensor data.
After resetting the engine light the light will not come on straight away and the car runs without problems for the rest of the day. Not until I start the car the next day does the light typically come on.
Is there a common power supply in the engine ECU for the sensor and heater circuit? But then independent of the other sensor? Hmm that's a long bow to draw.
The car runs fine and if the engine light does not come on at start up it will not come on during the journey. If the light comes on it will not go out until reset by the LEXIA.
User avatar
My Name: C3driver52

Guru
Posts: 1854
Joined: Sat Apr 30, 2011 8:40 pm
Model: C3 2002-2005, Original shape model
Year: 2003 (03)
Engine Size: 1.4 i
Fuel Type: Petrol
Mileage: 79984
Trim Level: VTR+
Gearbox: Manual 5 speed
DPF: No
LHD or RHD: RHD (UK)
Engine name: TU3 (75 PS)
Has thanked: 85 times
Been thanked: 65 times

Post

Ozvtr wrote: Wed Aug 12, 2020 2:40 am Is there a common power supply in the engine ECU for the sensor and heater circuit? But then independent of the other sensor? Hmm that's a long bow to draw.
The car runs fine and if the engine light does not come on at start up it will not come on during the journey. If the light comes on it will not go out until reset by the LEXIA.
Do you have access to SEDRE?

Use that to pull up the ECU connection diagrams. You can see the path power takes from the ECU, through which plugs and sockets to the sensors.

You may see a common plug/socket that gives you the point at which power supply is split between sensors. This could be your point of failure. It can often just be a case of unplugging the offending connection and reconnecting them 8-)

But if that doesn't work, you could back probe the plugs or pierce the insulation to check for the supply volts.
User avatar
My Name: Ozvtr

Moderator
Posts: 942
Joined: Wed Jul 10, 2019 4:31 am
Model: C3 2002-2005, Original shape model
Year: 2003 (53)
Engine Size: 1.4 (8v)
Fuel Type: Petrol
Mileage: 80000
Trim Level: Other
Gearbox: Automatic PRND
DPF: No
LHD or RHD: RHD
Engine name: TU3 (75 PS)
Location: Brisbane, Australia.
Has thanked: 36 times
Been thanked: 252 times

Post

The two O2 sensor heaters derive their power from the BSM and the returns go independently into the engine ECU. There is one common power line from the BSM which is spliced in the engine loom and then go to the two O2 sensor connectors. So if that was the problem then both O2 heaters would not work. If one of the wires were broken that would only effect that heater and not the sensor.
The O2 sensors are each a passive device generating voltage in response to the level of O2 in the exhaust stream. Both wires from each of the O2 sensors go straight to the engine ECU and are not connected.

The only single common point for the upstream O2 sensor and heater is the green connector on the top of the thermostat housing. Both the heater and sensor wires and only those wires got through that connector.

It is possible that the upstream O2 sensor and heater have a common ground inside the engine ECU but that now becomes an internal ECU problem and it is only conjecture and not fact.
User avatar
My Name: Ozvtr

Moderator
Posts: 942
Joined: Wed Jul 10, 2019 4:31 am
Model: C3 2002-2005, Original shape model
Year: 2003 (53)
Engine Size: 1.4 (8v)
Fuel Type: Petrol
Mileage: 80000
Trim Level: Other
Gearbox: Automatic PRND
DPF: No
LHD or RHD: RHD
Engine name: TU3 (75 PS)
Location: Brisbane, Australia.
Has thanked: 36 times
Been thanked: 252 times

Post

Ok, so I made a few mistakes and went off half cocked :oops:
screen 002.jpg
screen 003.jpg
This one occurred during warm up. Coolant temp is 41'. So a minute or two after start up
screen 004.jpg
This is about 5-10 minutes after start up. 80KMH zone is just down the road from work and is only a couple of kilometers long.

Looks like my major problem is the the heater. The other two are temporary faults and explain why I can read sensor data.
There is no code or data for the heater fault!? Clicking on the bar does nothing.
User avatar
My Name: Ozvtr

Moderator
Posts: 942
Joined: Wed Jul 10, 2019 4:31 am
Model: C3 2002-2005, Original shape model
Year: 2003 (53)
Engine Size: 1.4 (8v)
Fuel Type: Petrol
Mileage: 80000
Trim Level: Other
Gearbox: Automatic PRND
DPF: No
LHD or RHD: RHD
Engine name: TU3 (75 PS)
Location: Brisbane, Australia.
Has thanked: 36 times
Been thanked: 252 times

Post

Ok. So I have done a bit more poking around.
As you can see by the photo the heater circuit is virtually open circuit. So that confirms the "permanent heater fault".
lambda 003.jpg
There are four wires in the O2 sensor. 2 white 1 black and 1 grey.
The two white wires are the heater wires. The black wire will be 0 volts reference and the grey wire will be the O2 sensor output.

Next up is graphical data of the engine coolant outlet temp. The upstream (closest to the engine) O2 sensor and then the downstream (after cat) O2 sensor during a run.
lambda 004.jpg
The system will only capture 900 seconds (15 minutes) of data.
This was a trip "from cold" and almost straight onto the highway. As you can see the outlet temp climbs from about 20'C to about 90'C and then levels out to about 86'C. The interesting thing is the oscillations in the temperature as the thermostat opens and closes slightly to regulate the temperature.

If I turn your attention to the middle graph, this is the upstream O2 sensor output in volts. The range is from 0 volts to 1 volt. You will see that the output is stable up to about the 150-175 second mark. This indicates that the engine is in open loop mode. This means that the air fuel ratio is fixed to a pre-programed specification. After the 175 second mark you will see the engine go into closed loop mode. The engine ECU now adjust the mixture slightly rich then slightly lean, about once each second.
This all looks OK.

Now the downstream sensor...this is where it all comes unraveled! This is the bottom graph.
As far as oxygen is concerned, the catalytic converter is a bit like a reservoir. It will store oxygen from lean (oxygen rich) cycles and then give it off during rich (low oxygen) cycles. Now there is more going on in the catalytic converter than that, but we are only monitoring the oxygen. So what we should get is a relatively flat line at a point just over 0.5V.
Oh dear, that's not what I'm getting! LOL! You can see the downstream sensor almost mirrors the upstream sensor meaning the catalytic converter is doing nothing!
Hmm, in one of my other posts I found that 2 of the injectors were blocked. Meaning the two good ones were over compensating, and running a very rich mixture. Well I believe that rich mixture has coated the catalytic converter in soot and ruined it!
There is my "aging catalytic converter" fault!

Ok, but what about the temporary O2 sensor fault? Well the heater is open circuit so it's possible the sensor element is not in the best of shape either.From time to time it might not work for a few seconds then start working again. If you look, there are a few times around the 700 second mark where the voltage drops to 0 volts. And there are a few points around the 800 and 900 second marks where the voltage stays low for a few seconds then bounces up. These are points where the engine ECU might cry foul and throw up a code.

So what to do? The upstream sensor should be replaced. The catalytic converter...well...I have some tricks I want to try before I make a decision about that. Replacing it could prove expensive AND do I need to replace it? No MOT here in Qeensland and anyway there are tricks to make the ECU think the CAT is working.

Stay tuned!
  • Similar Topics
    Replies
    Views
    Last post