Following a recent P0171 code I'm kind of in a diy-mechanic/doctor/inspector/want-to-learn-more mode to see how my tin cat is actually purring. I have a background in electronics and have done a couple of engine rebuilds in the past (much simpler engines than my 2003 XKR). I also have a good quality multimeter and an oscilloscope available so I'm not really afraid to poke and probe around.

What I don't know is how the O2 sensors actually work? Is there a way to see they're doing what they're supposed to using an oscilloscope on a running car? If so, where would be a convenient place to insert the probes in?

Thanks,
K

 

Found a good article about this here: What the Home Mechanic Needs to Know about O2 Sensors

Now I just need to find where to access the raw signal from the sensors conveniently. Any suggestions?

 

As far as I know, the actual voltage for all 4 O2 sensors is available over OBDII, so a cheap ELM327 with Torque Pro will show this.

 

I've got those as well. Will have a look when I get a chance. Just wondering is it going to be fast enough to show the variations?

I'll take a look at the electrical diagrams and see if the raw signal is accessible somewhere. Perhaps directly at the ECM connector...

 

They should be accessible at the ECM connector, although calling anything accessible on these cars tends to be a bit of an overstatement.

P0171 means that the long term average fuel trim is too lean. If you watch the raw output voltage it will likely bounce around a lot but that isn't necessarily incorrect.

Aside from wiring problems where the voltage will be a constant high or low, the usual fail mode for an O2 sensor is that they become less sensitive with age and the average output shifts.

Since there isn't a glaringly obvious "good" and "bad" signal, the way to know if the sensor has gone wrong is to have a 2nd measurement for comparison (not always easy to do).

You could compare the left and right sides, but I'm not sure how you can tell if the sensor is bad vs an actual air leak on that bank.

 

I'll do a comparison like you suggested using my oscilloscope just out of curiosity.

I've already tried to hunt down an air leak with some success but the trim remains higher than I'd like. I replaced the cam cover gaskets and a bunch of breather hose o-rings. I also threw in a new PCV valve, oil filler cap and a dipstick plus cleaned the MAF sensor really good. A home made smoke test didn't reveal any leaks so I'm now trying to see other possibilities including clogged up fuel injectors and the o2 sensors.

 

You might want to look up "fuel trims". There are a couple of great tutorials on this forum. Search is your friend. The short version is that the 4 O2 sensors provide input into a computer-based fuel adjustment strategy on top of the base fuel maps coded at the factory, all for the purpose of maintaining an air/fuel ratio that promotes good driveability and clean emissions. It is the usual action, measure, then correct feedback loop. When those adjustments become too great (25%), a code is thrown because something is likely wrong. These adjustments should ideally be low if everything is in "perfect" running order. I would encourage you to check these fuel trims with your ELM device.

Best of luck, keep us posted.

 

+1

BTW The posted article is years out of date - be prepared to find the front O2s may be wideband and if so are current-based not voltage-based.

Use OBD II as the cars are designed for that.

 

You are absolutely right about that. Wasn't thinking strait, the sensors are current based.

 

Think I found the problem. At least one of them. I hooked up my ELM327 and Torque Pro and set it to graph all the O2 values it could read. There were only three values I could graph:

- O2 Sensor 1 Equivalence Ratio(alternate)
- O2 Volts Bank 1 Sensor 2
- O2 Volts Bank 2 Sensor 2

No data at all from the other two sensors. Not sure why?

Anyway. I let it idle to normal temperature and then drove for few miles while graphing the above values. Bank 2 showed activity depending how I was driving. Bank 1 however remained at almost 0V all the time. I had to replace the other sensor on this side a couple of years ago as it broke and threw a code. Time to replace the rest of the sensors I guess...


This is how the bank 1 sensor 2 (middle graph) looked like most of the time.



This was probably the most reaction I got out of the bank 1 sensor 2 (middle graph).

 

Seems you're trying to be a car computer (*) instead of looking at fuel trims (and if you like, freeze frame data).

(*) Possible, but harder. Read up on closed loop control if that's the way you want to go.

 

Sorry I should have mentioned I have been monitoring my fuel trims for a while already after I got the P0171. I have another thread about that here: https://www.jaguarforums.com/forum/xk8-xkr-x100-17/restricted-performance-p0171-182433/

Basically the bank 1 long term trim is elevated and I've already gone through hunting vacuum leak plus changed gaskets, o-rings etc etc. Did a smoke test too. No vacuum leaks found.

That's why I'm looking at other options such as clogged up fuel injectors, filter and the O2 sensors.

This particular sensor shows near no activity compared to the other side of the engine which does not sound right to me...

 

Old thread I know but I thought I'd update anyway just in case someone is reading this. The bank 1 downstream O2 sensor was indeed dead. Had it replaced and things returned to normal plus my LTFT dropped a little in the process... The cat is purring happy again

 

Back to enjoying the car