Technical Downstream O2 sensor voltage

[FirstTimeFiatDriver]

I checked the voltage of the O2 sensors with an OBD scan tool and the O2 sensor placed after the catalytic converter seems stuck at 1.275 V when the car is idling with the engine warmed up. Reving the car doesn't make any difference to the voltage, only the upstream sensor is affected.

I don't get any related DTCs. Is the sensor faulty?

 

[bugsymike]

I checked the voltage of the O2 sensors with an OBD scan tool and the O2 sensor placed after the catalytic converter seems stuck at 1.275 V when the car is idling with the engine warmed up. Reving the car doesn't make any difference to the voltage, only the upstream sensor is affected.

I don't get any related DTCs. Is the sensor faulty?

I stand to be corrected, but as I understand it the up stream sensor fluctuates as it tells the ECU to correct the fuel trim, whilst if it has been corrected then the downstream one is usually fairly static as fuel /oxygen /lambda is correct.

 

[FirstTimeFiatDriver]

I stand to be corrected, but as I understand it the up stream sensor fluctuates as it tells the ECU to correct the fuel trim, whilst if it has been corrected then the downstream one is usually fairly static as fuel /oxygen /lambda is correct.

You are right. The downstream sensor should be stable with little fluctuations when revving abruptly. But I don't this it should be stable around 1.275 V, but around 0.45V.

Furthermore, today the sensor's voltage fluctuated widely when revving, so maybe something is going on either with the sensor or with the catalyst.

 

[bugsymike]

You are right. The downstream sensor should be stable with little fluctuations when revving abruptly. But I don't this it should be stable around 1.275 V, but around 0.45V.

Furthermore, today the sensor's voltage fluctuated widely when revving, so maybe something is going on either with the sensor or with the catalyst.

What were the revs, I agree around idle .5volts would be expected, also was the engine fully warmed up.
One of my sisters cars had an issue with the heater wiring at the downstream Lambda sensor on a Renault, car ran fine, but admittedly it did show the error code.
Annoyingly her garage were going to change the upstream Lambda sensor as they pointed it out to her and were saying it was seized and they may need to replace the catalyst!!!!
Luckily she spoke to me before they started.

 

[FirstTimeFiatDriver]

@bugsymike

In both cases the engine was fully warmed up. The catalytic converter's temp was 447 °C the first time when the O2 read 1.275 V and 532 °C when it fluctuated widely, surely between 0.2 - 0.8 V, if not even more.

The car was idling when I got the values, so the revs were lower than 1000. I revved the car up to 2000 - 3000 revs to check how the reading would change and that caused wide fluctuations to the voltage of the downstream sensor which I'm sure it's not normal.

Both lambda sensors were changed as a pair about 20.000 km ago.

I'm having the injector's of the LPG system replaced today and the LPG reducer repaired with a repair kit. The fuel mixture was rich due to faulty gas injectors with fuel consumption reaching around 20 l / 100 km. I don't think though that the rich fuel mixture had any effect damaging the sensors. I guess the one before the catalytic converter is more vulnerable.

 

[Pugglt Auld Jock]

I stand to be corrected, but as I understand it the up stream sensor fluctuates as it tells the ECU to correct the fuel trim, whilst if it has been corrected then the downstream one is usually fairly static as fuel /oxygen /lambda is correct.

This is one I like to check using MES where I can graph it. Output should be pretty much straight line - with cat fully warmed up. I've found the actual voltage, which I would assume would be .5 volts, often varies a bit - often on the higher side, so maybe .7 volts etc - I think if the voltage is somewhere near "sensible" and, more than anything else, straight lines the graph then I don't worry about it. I'd always graph the upstream sensor too which I would expect to see flip flopping between about 0.1 and 0.9 volts although I wouldn't be too worried if the range was not quite reaching these figures as long as it was switching at least once per second - it'll switch faster at higher revs than at idle.

 

[bugsymike]

@bugsymike

In both cases the engine was fully warmed up. The catalytic converter's temp was 447 °C the first time when the O2 read 1.275 V and 532 °C when it fluctuated widely, surely between 0.2 - 0.8 V, if not even more.

The car was idling when I got the values, so the revs were lower than 1000. I revved the car up to 2000 - 3000 revs to check how the reading would change and that caused wide fluctuations to the voltage of the downstream sensor which I'm sure it's not normal.

Both lambda sensors were changed as a pair about 20.000 km ago.

I'm having the injector's of the LPG system replaced today and the LPG reducer repaired with a repair kit. The fuel mixture was rich due to faulty gas injectors with fuel consumption reaching around 20 l / 100 km. I don't think though that the rich fuel mixture had any effect damaging the sensors. I guess the one before the catalytic converter is more vulnerable.

I didn't recall you mentioning LPG system, I know nothing about that on vehicles, only very basic systems on stationary engines.

 

[FirstTimeFiatDriver]

@bugsymike

I had taken the O2 voltage readings with the car running on petrol. There shouldn't be any difference when running on LPG regarding the oxygen sensors. By the way the car has a factory installed LPG system.

@Pugglt Auld Jock

What's the reading of the downstream O2 sensor when turning on the engine (but with it being fully warmed up)? I got the 1.275 V reading again today. The O2 sensor was stuck at that value and didn't change when revving the engine. The coolant temp was around 90. Is it possible that there is a delay for it to start working? As far as I remember the upstream sensor was reading 1.275 V too for a while (for a few seconds), but it started responding. On the other hand the downstream sensor stayed at 1.275 for over 1 minute, maybe more.

 

[bugsymike]

@bugsymike

I had taken the O2 voltage readings with the car running on petrol. There shouldn't be any difference when running on LPG regarding the oxygen sensors. By the way the car has a factory installed LPG system.

Generally modern O2 sensors have an extra two wires (often white) that power a heating element in the sensor to react faster from cold.
Although I would expect an error code if that failed.
I understand re running on petrol, it is just that LPG less popular here so never got involved with it.

 

[FirstTimeFiatDriver]

Well, what's the reading of the voltage of the sensors while they haven't fully warmed up and the car runs in an open loop? Is it 1.275? Does this reading mean the car runs on an open loop?

What's the deal when the engine is already warmed up? Do the sensors pass through their warming cycle?

 

[bugsymike]

Well, what's the reading of the voltage of the sensors while they haven't fully warmed up and the car runs in an open loop? Is it 1.275? Does this reading mean the car runs on an open loop?

What's the deal when the engine is already warmed up? Do the sensors pass through their warming cycle?

Surely your diagnostics will show if Open or Closed Loop running.
If Cat that hot (447 Degrees C, then both sensors should be at normal range, sorry I have no data re that reading is.

 

[FirstTimeFiatDriver]

@bugsymike

I'm using a generic ELM327 scan tool and some free Android apps. I haven't seen them reporting anything about an open or closed loop.

 

[bugsymike]

@bugsymike

I'm using a generic ELM327 scan tool and some free Android apps. I haven't seen them reporting anything about an open or closed loop.

Many on Forum use MultiEcuScan and can give good advise on it, I use MaxieEcu on a laptop as it works with my Fiat Scudo that uses the Peugeot engine, but also on my other Fiat diesels.

 

[FirstTimeFiatDriver]

It would be nice if someone could share what readings the MES tool gives.

 

[Pugglt Auld Jock]

It would be nice if someone could share what readings the MES tool gives.

Sorry I've not been back before now but we were out at the grandchildren until late last night and this morning I've been trying to sort out more of the problems my brother left behind him when he died - The solicitor must be rubbing his avaricious hands together!

Anyway, back to these sensors. A conventional oxygen sensor should be capable of outputting between around 0.1 and 0.9 volts - think of it as a wee battery powered by Oxygen. The more oxygen in the gasses the more voltage it can produce.

I would be monitoring this by graphing it which gives you the "whole picture" in a way a meter can't. So, I like to bring up both upstream and downstream sensors at the same time. Of course this needs to be done with a hot engine so coolant temperature at normal running temperature - although, with built in heaters in most O2 sensors now, the sensor should settle down to a meaningful reading shortly after start up - but I'd not feel happy until the engine is "proper hot" and I can be sure it should be in closed loop.

If the cat is working properly then it should reduce the exhaust gasses to largely H2O (water) and CO2 and largely eliminate the switching of the downstream sensor in the process. In theory I think you should see a steady 0.5 volts at the downstream sensor with only minor fluctuations. however my boy's Punto ran around 0.8 and seemed very happy and passed it's MOT. The important thing is that it's straight lining rather than the precise voltage it's generating.

Here's a video which may help:

If the cat fails then the downstream sensor will start to mimic the upstream sensor but you say your downstream sensor output is steady showing 1.275 volts? which would indicate an excess of oxygen - possibly because of a leak somewhere in the system upstream of this allowing outside air to be drawn in. I've never experienced this so I can't think of anything more to say right now. Good luck.

 

[FirstTimeFiatDriver]

@Pugglt Auld Jock

Thanks a million for the time you took posting this detailed reply! No need to be sorry, it's nothing urgent anyway!

With the voltage at 0.8, λ (lambda) is still close to 1, as far as I can tell by looking at the diagram bellow, so it's not a surprise emissions were within specs.

By the way, good luck sorting out all your familial matters. I wish you soon to be free of the need to cooperate with any solicitors, avaricious or not!