Technical Help or advice to diagnose o2 lambda sensor issue

[t5nel]

Hi All,

I am having trouble working out the issue with my 500 - The car starts ok and after about 10 seconds begins to idle like a dog. It is now reporting a P0130 code and I can see from live data that the pre cat sensor is registering a consistent 0V. I have swapped the O2 sensor for a new Bosch one and STILL get the same issue 0V P0130 and terrible running. Whilst I guess it is possible that I have 2 duff sensors I am starting to thing there may be an electrical (fuse?) wiring or ECU issue.

Looking for help in tracing whether the wiring is good from harness to ECU and any other suggestions really. Is it obvious which pins I need to test in the ecu connector etc.

For a bit more context...

Recently bought a 2017 500s for my daughter as her first car. It has done 50k miles and is reasonably tidy. It looked and drove fine when I bought it. I ran around in it for about 400 miles before handing it over... Almost immediately the thermostat plastic spigot snapped and the car overheated. We limped to a garage, coasted engine off and then waited for AA. They did a roadside fix which involved removing the plastic spigot remnant and replacing with a metal unit. The car got me home though it sounded rough. Obviously to fix the issue they had to remove the battery/tray and ECU. I have replaced the radiator, thermostat housing and o2 sensor but the car is still running just the same :-/

Thanks
Tim

 

[Eltezz]

This may not be the issue but… the one thing the 1.2 engine doesn’t like is overheating. This can lead to blowing the head gasket which will lead to coolant or oil mixing with the fuel. This could potentially cause the poor running and P0130 O2 sensor readings.

You would need to either borescope the cylinders or do a pressure or leakdown test to rule it out. If you don’t have the kit then a garage could do this for you.

 

[t5nel]

Thanks for your comments. This was my first concern however…

Pretty sure HG is ok. AA did a sniff test due to
Rough idle and there are no other signs of oil in coolant or vice versa. All plugs when pulled were sooty and none seem steam cleaned.

I will try a compression test as it is easy

I just is not think a working o2 sensor would ever give 0V which makes me suspect an electrical issue.

If others have seen the same due to HG failure then that would be useful to know.

 

[Eltezz]

My understanding is that an O2 sensor will give a 1V output for a very rich mixture and a 0V output for a very lean mixture so reading 0V doesn’t necessarily mean it is duff, especially as you have tried two now. I think I have a pin out for the ECU somewhere, I will post it up if I can find it. Might help you check wiring continuity.

 

[Mike1alike]

Hi and Welcome to the forum!
Since you’ve fitted a new Bosch sensor and still see a constant 0 V, the problem is very likely not the sensor itself. The ECU normally biases the lambda signal to about ~0.45 V when the sensor isn’t producing a signal.
A quick check: with ignition on, look at the live data and then unplug the pre-cat O₂ sensor. If the wiring and ECU are OK, the voltage usually jumps to around 0.45 V. If it stays at 0 V, that suggests the signal wire is either open or shorted to ground.
Given that the battery tray and ECU were removed during the repair, I would also carefully check the ECU connectors and the lambda wiring in that area for a pulled pin or trapped wire.

 

[Eltezz]

Pinout if you need it...

 

[t5nel]

Hi and Welcome to the forum!
Since you’ve fitted a new Bosch sensor and still see a constant 0 V, the problem is very likely not the sensor itself. The ECU normally biases the lambda signal to about ~0.45 V when the sensor isn’t producing a signal.
A quick check: with ignition on, look at the live data and then unplug the pre-cat O₂ sensor. If the wiring and ECU are OK, the voltage usually jumps to around 0.45 V. If it stays at 0 V, that suggests the signal wire is either open or shorted to ground.
Given that the battery tray and ECU were removed during the repair, I would also carefully check the ECU connectors and the lambda wiring in that area for a pulled pin or trapped wire.

Yes this was my feeling too. It is difficult to unplug the sensor due to location! But I will try this later if I get a chance.

After posting this yesterday I did start looking at the ECU and... there is a short pin in there.

 

[t5nel]

Pinout if you need it...


View attachment 482984

This is SO useful and I think highlights my issue. See attached ECU connector. I had noticed the short pin yesterday when I took this pic (after my initial post). one of the black 'supports' is also snapped off/broken.

I guess the question now is...Is the ECU toast, is it likely the pin is pushed in (it looks that way) does this haoppen often to these and is there a repair service...

Do you guys agree this looks a likely culprit?

 

[Mike1alike]

Do you guys agree this looks a likely culprit?

It surely does! It's the second pin on the row above '13', right?
That totally does no contact with the plug and most likely is the culprit.
No need to search for a repair service just yet, most likely nothing is damaged. Just pin pushed inside. What you need to do is pull it back, use some tweezers or something. It happened to me once and everything was in working order after I put the pin back in position.
With ECU plugs is very important you come straight with the plug when plugging in, having all those pins there. That one was caught by an edge of the plug, that's why was pushed in. So make sure all pins stay straight and put the plug straight then push it in slowly, no force needed.
The support being broken is not a big deal, one of mine is so for many years ago. Plugs have the isolation at the edge that keeps them in place, mine never moved out.

 

[t5nel]

It surely does! It's the second pin on the row above '13', right?
That totally does no contact with the plug and most likely is the culprit.
No need to search for a repair service just yet, most likely nothing is damaged. Just pin pushed inside. What you need to do is pull it back, use some tweezers or something. It happened to me once and everything was in working order after I put the pin back in position.
With ECU plugs is very important you come straight with the plug when plugging in, having all those pins there. That one was caught by an edge of the plug, that's why was pushed in. So make sure all pins stay straight and put the plug straight then push it in slowly, no force needed.
The support being broken is not a big deal, one of mine is so for many years ago. Plugs have the isolation at the edge that keeps them in place, mine never moved out.

Yes agree - it is pin 26 which is highlighted as related to precat lambda sensor. I will try and pull the pin back later.

This rough idle happened IMMEDIATELY after the AA repair. It was raining heavily when he was working and was dark so I think he put it back on clumsily and maybe forced it., I do not think that I have much claim against them though...

Lets hope I can fix it. can the ECU be opened / plugs replaced if I cannot pull the pin? Replacing the whole ECU seems expensive

 

[Mike1alike]

can the ECU be opened / plugs replaced if I cannot pull the pin?

There are two tipes. One the case has screws and it's very easy to open, second has the case glued. This one too can be opened but you have to insist and also work with great care. Problem is the wiring inside is very thin, most ECU have the circuits sitting in gelo to protect the thin wiring against vibrations and all. So would be better to avoid opening it up. Just try and pull it out, it should work. To test it, I think there is another pin on ECU (I don't know which) that you can do some resistance measuring and know if it's ok. But anyway, having it connected and engine running or diagnose plugged in, you will know for sure if it works.

 

[Steveetee55]

I had problems with my twin air and touch wood i think it was an iffy connector on the smaller ecu main plug i found it by running a continuity test with a multimeter across either end of the wiring loom connections and wiggling the loom continuity keep going on and off on the multimeter i've cleaned the plug and socket for now with contact cleaner if if reoccurs i will try and squeeze up the offending connector joint

 

[Mike1alike]

To test it, I think there is another pin on ECU (I don't know which) that you can do some resistance measuring and know if it's ok.

It is actually between the pin and ECU's ground you can measure. ECU's ground is the aluminium plate at the back. Have the multimeter set on continuity and there should be about 450~550 Ω, might vary on ghe model. You can start with that if you have a multimeter at hand, before pulling the pin back. Then you'll know for sure it is not damaged.

 

[t5nel]

Well I got out there, pulled the batery tray and ecu and got to work with some (too large) pliers!

I managed to imporve the pin layout and reassembled. after clearing codes I get a sane and changing voltage for the B1S1 sensor AND my car idles!!!

Thanks both of you for the information - it stopped me from giving up .

Pics showing result.

 

[Mike1alike]

So it's fixed and in working order now, right?

 

[t5nel]

Yes!

Just checking that there are no leaks on radiator or thermostat as I replaced these. The rough idle and poor performance are gone. It did need the codes to be cleared before it ran in ‘closed loop’ feedback from the o2