Technical Intermittent MIL

Currently reading:
Technical Intermittent MIL

Once fully warm, a minute at tickover and then rev up to clear it and let it settle for a minute again so we can see what's happening at tickover. Do a graph trace of rpm, O2 1 and O2 2 if you can

In the meantime, I'd start checking exhaust for leaks, put a well gloved hand firmly on the exhaust end with engine running and listen, look for black soot along the exhaust particularly at joints. Think about any other faults occuring like misfiring that may affect air/fuel tuning
 
Last edited:
Looks like your post cat lambda is shutting off and going beyond limits of too low when you are anything above tickover rpm

So your obd is dead right- signal low

I'd check out the O2 sensor heater circuit- see if there's low resistance continuity in the heater with ign off and see if 12v and earth are being supplied to it with ign on and engine running

And check for exhaust leaks. It looks like your O2 sensor is cooling off at higher rpms so much that it turns off
 
Last edited:
richydraper: just question - in the first message you wrote, that you have catback system exhaust system now in the car? So cat is now the last part of the exhaust? And the second o2 sensor is where?
 
there is 3 parts to my exhaust system. Manicat, intermediate pipe and rear section including back box. 2 days before i bought the car it had a new 'mild steel' intermediate and rear section fitted.

from what i can make out (from parts diagrams, i haven't actually physically looked yet) the post cat lambda is just before the flange from manicat to intermediate pipe.
 
So you think, that you have standard exhaust system with cat in manicat just behind the engine? It is not possible that you have cat in the rear part, because you wrote that "...it had a cat back mild steel system... "?
 
Ok, it is clear now.
But readings from after-cat are very strange, looks like cat is not working... but I'm not sure...
 
thats my worst fear... but i have the Post-Cat Lambda Fault (P0136) so will be checking operation probably this weekend when i can get her over a pit at work to confirm or deny.

The damn thing seems to work fine at revs, as the graph trace shows... but id be expecting to see the same trace at tickover if the sensor was good.
 
If you will do new log of livedata from o2 sensors, use higher sample speed as 120-200 samples/minute,,, 60 samples/minute (1/ses) is not enough to see all details.
 
unable to give any more bud... 60/sec is all FES will do
Untitled.jpg
 
Your post cat O2 sensor is working ok, it's switching hi /lo around every sec when you're at tickover BUT it's doing the same as front sensor and that normally means the cat isn't doing anything. But you'd get the same effect if you wrongly put both O2 sensors say side by side so they're both sampling the same thing

What's odd is that the post cat O2 shows abnormally low reading- off the chart- when you are above tickover- as if the sensor is switching off with low temps

So these are why the OBD is saying bad O2 sensor signal- low and P0420 bad cat efficiency.

You could ignore it of course, a garage would be able to do an emissions check for you if you're worried as the front lambda does all the hard work of the A/F mixture signals and the garage check would spot if the cat is working acceptably. So if your car is running right and emissions are good and it's just an O2 sensor problem then it's largely up to you whether you try to fix the puzzled OBD

Or maybe fit one of these
http://www.bigdaddiesgarage.com/mini-cat-cel-fix.html
 
Last edited:
I was thinking about this problem a little bit. For right function of cat, there must be fuel mixture ok in engine or there will be problem. Fuel mixture is control by pre-cat o2 sensor.
And data from pre-cat o2 sensor are little bit strange (from your csv file).
The lower limit is only cca 0,24V and should be cca 0,1-0,15V.
And also when after cat o2 sensor reached 0V, pre-cat o2 sensor reached only 0,1V - marked on att. image.
You can try to do this test:
on 5th gear, speed cca 50mph, then full throttle up to 90mph, then release throttle and soft breaking and save rpm, o2 sensors data, to see this: http://merkur_sound.sweb.cz/lambda2a.png
If the pre-cat o2 sensor will not reach 0V, like in my car, then problem is probably in this sensor.
 

Attachments

  • ef.PNG
    ef.PNG
    89.7 KB · Views: 45
MIL is back as of the 26th haven't had chance to hook up FES due to work and the little 'un wanting to go to the park!:cool:(y)

My money is on P0420 so ive been having a look round for Lambda sensors it all seems a little expensive but heres what i found

Fiat 46750243, 46481568
NTK OZA532-A2
Bosch 0258006193 0258006072

I've found a variety of Universal 4 Wire Lambdas that are MUCH chaper (60%-70% cheaper) too ranging from 4-16 ohm heater resistance circuits... will this matter? Whats the 'proper' heater resistance figure?

from what i can make out the bosch 'direct fit' units have a 600mm lead wire listed for the sensor, lead wire being from the sensor mating surface to the end of the multiplug, so i could get any suitable universal unit and chop and solder the wiring to suit. i also found that these sensors are 'body isolated' types so im assuming that means they have their own ground wire and its not grounded through the exhaust.

does this make any sense or am i slowly loosing it?
 
4 wire universal is fine,heater resistance will be fine.most problems are through people making really poor solder joints,bear in mind signal is less than 1v.grey wire is sensor earth so they don't earth through exhaust.look for quick signal response,some universals can be sluggish
 
Last edited:
Back
Top