Oscilloscope Test Precat O2 Sensor

[Toolaa]

I think my gas milage is really lower than it should be and I trying to decide if replacing the Pre-cat O2 sensor is going to make a difference. I'm not getting any codes but I don't know if it's ever been replaced.

I tested it with an Oscilloscope and the waveform looks really rough lots of spikes and noise in the signal. The only thing Im comparing it too are examples online from other cars.

Voltage seems to be ranging between .2v-.8v

I tested the response time by forcing lean and rich conditions and the voltages changed as expected. The only thing I don't know is if a 5 wire sensor works the same way as the old 4 wire sensors.




Is there anyone who has done this before and knows how this should look.




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[old guy]

The wideband sensor acts considerably different than a standard narrow band sensor as shown here. You really can't compare the voltage reading between the two.

And if your voltage readings are between .2v and .8v you are probably monitoring your rear O2 sensor.

[Toolaa]

I'm sure I was connected to the Pre-cat sensor. I back probed the connector with 5 wires.



So When I got that waveform It look sort of normal.

Where did you find that chart. Need to do more research I guess.


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[old guy]

Clicky click. Just Google wide band versus narrow band sensors and you can find a lot of info. For what it's worth I'm going through the same thing right now. I have been messing around with several different "used" wideband sensors and getting mixed results. I just ordered a new sensor this morning from here: Clicky click. Cheap enough for a Bosch sensor. I'll let you know how it works out.

[Toolaa]

Thanks for the link. Man I didn't realize how different they were.Well looks like I was probing the pump signal from the ECU, which is supposed to fluctuate.

What is really strange is that when I added propane to the intake the average voltage increased and when I unplugged a vacuum line to create a lean condition the voltage dropped almost like a narrow band sensor would respond. That's what threw me off.

There are ways to test these. I'm going to try again this weekend and share any insights as well. I found some good links too but its getting late.


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[ADCS]

I would log the sensor output via VCDS to see what it is doing under load instead of at idle.

[Toolaa]

I would log the sensor output via VCDS to see what it is doing under load instead of at idle.

Do you know if there is a specific VCDS measuring block which displays the actual sensor voltage? I thought it just displays the calculated lambda value.


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[old guy]

Do you know if there is a specific VCDS measuring block which displays the actual sensor voltage? I thought it just displays the calculated lambda value.

Logging sensor voltage isn't going to work with a wideband sensor. And you are correct. You will be looking at Lambda readings. Here's some decent info on the Bosch wideband sensor operation: Clicky click

[Toolaa]

I'm wondering if you replaced your Pre-Cat yet and what were the results.

Here is where I stand: MPG seems crappy 18.4 mpg last tank Mid-grade. 60% Hwy driving.

Today I went through VCDS and checked and logged most of the relevant mesuring blocks

Logged 031 through several 2nd and 3rd gear pulls from 1500-6000 rpm and A/F ratio was very consistant 14.7
Except when I would downshift and engine brake then I would see that the ECU was expecting 15.4 but actual was 29.3 (Lamda 1.99) I'm not sure if that is normal behavior??

Block 034 Pre-Cat Aging result was 1.265 (Still between 0.5 and 2) so technically good but on the low side

Blocks 036 and 037 for Post-Cat Passed

Only other problem I found was Block 046 Cat Coversion test. Result was 1.94 "Cat B1 not OK" (<0.50 = Fail)

I'm not sure if I should just bite the bullet and replace the Pre-Cat O2 Sensor or the Cat. (or both)

[customa4]

Clicky click. Just Google wide band versus narrow band sensors and you can find a lot of info. For what it's worth I'm going through the same thing right now. I have been messing around with several different "used" wideband sensors and getting mixed results. I just ordered a new sensor this morning from here: Clicky click. Cheap enough for a Bosch sensor. I'll let you know how it works out.

Damn OG, you have some serious bargain shopping skills!

[old guy]

My replacement sensor is due tomorrow. Unfortunately I am on the other side of the country right now (Sunny San Diego) and won't be back until Friday. I'll stick it it Saturday and let you know how it performs. I'm surprised to hear that your A/F stayed consistently at 14.7 during a pull. Mine usually drops to the low 13's or high 12's under load. Sounds almost like you are stuck in an open loop condition. I do get some spikes when off throttle but not to the extent that you are reporting.

[Toolaa]

I looked closer at the data and some other logs. Under heavy load WOT and boost 6-8.5 psi the number A/F ratio ranges between 13.7 to 14.7 The specified ratio and the actual ratio are within 0.1. So I took that as a good sign.

There are spikes when I take my foot off the gas after a hard pull with the engine in 3rd gear manual the revs will stay high. Load drops to 14%. Injector time on cycle drops to 0. MAF ~10.5g/s and A/F jumps to 29.3, but the ECU is calling for 15.4 A/F ratio. These values will hold very steady as long as I continue engine breaking. I repeated this pattern several times.

I would like to do some more pulls on a longer stretch of road. It's time like this that I want to spring for the full version of VCDS for some better logs. I'm also thinking about pulling the sensor out and checking the readings using the technique where you wrap the sensor in a rag sprayed with brake cleaner to force a rich reading.

I may just pull the trigger in the Pre-cat O2. Amazon has the same one for $74 free shipping. I'll keep an eye out for your progress.


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[old guy]

If you totally let off of the gas pedal and coast down in gear the ECM cuts the fuel completely so a high A/F ratio shouldn't be a concern. You can't get a 15.4 since there is no fuel to get there.

And I don't think you will learn anything with a rag and brake clean. the O2 sensor has to be at the correct operating temperature in order to operate properly. These are not conditions you can duplicate out of the cat.

[k909068]

If you totally let off of the gaS...

Sorry OP, not to hijacking your thread.

OG, Is post-cat O2 sensor reports signal to ECU to adjust A/F ratio, or it just monitors the efficiency of cat?
Having these codes after scan, debating on replace the O2 senser.
17526 - Oxygen (Lambda) Sensor Heating; B1 S2: Open Circuit
17522 - Oxygen (Lambda) Sensor; B1 S2: Internal Resistance too High

[old guy]

B1S2 rear sensor simply monitors the cat. It has a heater to maintain the proper operational temperature. Sounds like yours is bad and needs replacing.

[Toolaa]

^^Agreed. You can actually test the heater circuit of the sensor with a multimeter. Just disconnect the connector from the firewall. I think the heater wires are white and grey. You should see resistance. If you get an open circuit then it's either a broken wire or the actual heater is fried. Good thing is that this is the cheaper of the two sensors.


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[k909068]

Thank you guys!!!