Hello im drivng fabia mk1 1.4 aua engine.The problem is high fuel consumption.Scanned with Vcds no fault codes stored.Replaced the temperature sensor and thermostat.Car warm up an hold 90 degrees.Pre cat Lambda sensor is showing only 0,434V in idle and dont change.STFT and LTFT is always show 0.Thats not normal.When i unplug the o2 sensor the readings is same on group 33 on engine control module.But when is unplugged have a fault for o2 sensor signal and heating.When i plug it again not codes but ecu ignores it.Is this faulty o2 or not?

I guess that if you've unplugged the sensor, you have seen no problem with any broken wires in that area?

How many km has the car travelled?

 

No broken wires.car is about 200 000 km on dash

I think I would try another pre-cat sensor (possibly a recycled, scrapyard one) to see if you get different readings with that.

A constant (or almost constant) 0.45V is OK, but yours is a little lower than that on the rich side, and as you say, the fuel trims staying at zero indicates there's something wrong.  If wiring is all OK, that seems to leave only a sensor or ECU fault. Trying another sensor may help to see which of these two applies.

thanks for reply,it hink that precat o2 sensor is wideband and its range is 0-5v,but im not sure.When ignition is on but car is not started group 33 on VCDS show 8V.When start the car it goes to 0.434-0,450V.I think it s need to fluctuate when increase rpm.And another think is what other sensor data is used to get car in closed loop operation mode.Is temp sensor on Map is used?I can see catalyst temp 40 degrees.How its measured when there is no sensor on it?Is it possible ecu seeing that cold temp on cat and think is cold and ignore the o2 sensor and stay in open loop mode ?

Yes, with cold engine I see voltages around 6V at first, but this falls fast as the sensor heats itself up, stabilising at about 0.45 within 30 seconds approximately (in summer anyway). I think this 0.45V is maintained through all operating conditions, although sometimes there are brief excursions away from that value with big changes of acceleration etc.

 

Coolant temperature >75C and intake air temperature (via MAP temperature sensor)  <90C are required for fuel trim values to be adjusted, as far as I can see. 

 

Catalytic converter temperature is estimated according to the relationship between the energy required to maintain the lambda sensor's working temperature (duty cycle of heater current) and the known properties of the heater (PTC, positive temperature coefficient, type) within each sensor, I believe.  The less energy the sensor heater requires electrically, the hotter the exhaust gases must be. The ECU effectively measures the resistance of the heater, using that as a temperature sensor.

then i can have issues with labda heating resistance.This weekend i can try with another o2 sensor and see if it learn some values on trims.When it is in idle it must be save STFT.Thanks for your help

At ambient temperature, the resistance of the pre-cat lambda heater (pins 1 and 2 on connector) should be in the range 1-5 Ohms, I think.

very useful info,i can measure it with multimeter

Check the disconnect plugs at the front of the engine under the thermostat housing as they are know to corrode and give false lambda readings

i ve already check the plug,it is in good conditon,no corrosion

At 200k I'd be inclined to suspect the sensor is unreliable, it's probably pretty heavily fouled, you could try cleaning it but getting it out in one piece might be tricky.

I changed one recently that I was going to try and clean but it was so stuck that I had to snip the lead off and use a deep impact socket, even then it had me shaking on the end of a long bar. Good luck.

i found another o2 sensor for test,and will post the results here.I think it must run like in this videohttps://www.youtube.com/watch?v=FFsfkgYtqeY

Here's some data logged from my (2003) 9N Polo with BBY code engine (updated AUA). Link.

The first block of data is while I had a dead post-cat lambda sensor, but from about line 394, I believe everything is working normally with a new rear sensor fitted. Right at the end of the data, I have copied a few columns of data and then plotted rear sensor output voltage against engine load and both sensors' voltage against time for that journey. As far as I know that is normal behaviour. I think the front, pre-cat sensor was recently replaced a short time before this log. The ECU adjusts fuelling dynamically to attempt to maintain lambda=1, which is scaled to show 0.45V by this sensor/ECU combination.

 

Can you log similar data before and after changing pre-cat sensor?

Edited 27 January, 20179 yr by Wino

i can do log file,but what i see on your log that is normal operation mode 0.450V for precat sensor.Mine also works in this range.My wrong tihnking that is working range is 0-5v.Now i think that have a another problem not from precat sensor.What can cause fuel trims to stay 0?

Check that your intake air temperature isn't crazy-high? The thermostat in the airbox which maintains the blend of warm air from around the exhaust manifold, and cold air from the front of the car fails sometimes, causing all the air to come from around the exhaust manifold.

i will check.Intake air temp sensor is in map sensor i think?Do yo have fault codes for b1s2 sensor before replacing?

Yes the MAP sensor is really two sensors, pressure and temperature.

I think there was a code when I had B1S2 failure (after moving it from a broken exhaust to a replacement, it was OK before that). I only found the problem after doing an autoscan for another reason, no fault light appeared.

Checked the air box termostat and working good.Cleaned Map sensor working good too.Is it possible precat sensor failure but no Cel on dash?Sensor that i find for test is not compatible with my car.I have a one time -5% STFT when car is warming.Then i cleared trims and now again show only 0.I think to replace precat with new one because im out of idea.I think it must be lazy but not enough to ecu to get a error code....

Fixed!Replaced precat o2 sensor and now both fuel trims working.Now im testing fuel consumtion

Thanks for letting us know what worked.