help interpret live data for post cat o2

[steveo3002]

mk1 focus - got a p0420 code , dud cat /sensor/bad fuel

i grabbed some live data on the pre cat o2 it cycles up and down as i expected , post cat 02 reading is pegged at 1.27volts , doesnt budge a bit regardless of idle /driven

does this suggest post cat sensor is broken ?

[GreenV8S]

Definitely a bad reading. Can't guess whether that's due to a faulty sensor, wiring fault or faulty driver, but a faulty sensor is reasonably likely. It would be possible to test for a wiring/driver fault at the connector given the right tools and knowledge but also reasonable just to swap the sensor as long as they aren't too expensive.

[steveo3002]

thanks ^

ran it from cold this morning , reads 1.27v instantly from cold ...so yeah suggests to me sensor or related wiring

[steveo3002]

right , bosch post cat sensor fitted

lights cleared and came back on within 20mins

post cat 02 reading is flickering around 0.1 volts or so...does move a little

what now ?

[Belle427]

I would check all your wiring connections are ok first.
Are you happy with that voltage given, does it seem to be moving at all or is it stuck?
if all is well from memory it should sit at roughly 0.45 volts if the cat is happy.

[steveo3002]

its moving gently around 0.1....maybe 0.08 -0.14

certainly not 0.4 v

im told a dead cat would have sensor 2 going up and down in the same pattern as sensor 1 , not doing that?

[Belle427]

Normally yes.
No major leaks on the exhaust near the post cat sensor?
Can't explain that value really, even if the cat had partially collapsed I'd expect to see some more movement.
Some ford's in my experience don't like aftermarket sensors but bosch should be ok.

Edited by Belle427 on Monday 6th February 06:34

[steveo3002]

100s of these still roaming about ,hard to believe theyre all using genuine parts

i dont know what to say , saying more of a sensor /wiring fault to me

what do you think? see if a mot center will run an emmisions test and if its bad on emmisions then its the cat , if good then something else

[Belle427]

What are the fuel trims doing when it's running?

[steveo3002]

long term was 8 (dont recall if +/-) short term was 4

should i be thinking

new sensor dudd /wrong

needs cat anyway

or wiring issue meaning sensor 2 isnt giving accurate reading

[Belle427]

Can't swap post and pre cat sensors around?

[steveo3002]

nah theyre different

[steveo3002]

mot center tested the emissions today , cat is fine so i can get an mot with the light turned off

would still like to fix it ....so must be sensor or the wiring ? what now

[Belle427]

Maybe confirm wiring is sound from ecu to sensor?
just a thought but have you verified you have voltage on the sensor heater wires?
Clutching at straws really but worth checking.

[steveo3002]

yeah is starting to look like a wiring problem

[Belle427]

Be careful if you do start continuity testing though, here is a good video on it.
https://youtube.com/watch?v=xfohKS-HFGg&si=EnS...