bit of help with car diagnostic live data please

[steveo3002]

got an old mk1 focus , still on orig ford cat

today i get the code -p0420 cat problems

so i dont have much idea here , but got up some live data , o2 sensor 1 is cycling up and down as i expected , sensor 2 is pegged at 1.2 v and no amount of revving /driving moves it

does that suggest its the sensor ?

 

[Chippie]

I’m no expert….

Disconnect sensor 2 then check the ecu is saying something else…you could jury rig a supply to it from the ecu and measure the voltage coming back from the sensor, if it don’t move it’s probably ********

 

[m_c]

I hardly deal with petrols, but sensor two should occasionally cycle.
If it's permanently showing 1.2V it could be the sensor, or if it's only once warmed up, it could be the cat isn't doing what it should be.
At that age, it could be the pre-cat sensor isn't as accurate as it should be (they do age and become less sensitive), so although it's showing valid readings, they're not accurate and the ECU is over fuelling.

Normally I'd stick it on an emissions tester to get an idea as what is actually going on.

 

[tigdlo]

If it's a 4 wire sensor, which will be narrow band, then it should read 0.1v to 0.9v, if the ecu is reading 1.2v then you likely have a faulty sensor or wiring fault.

The post cat sensor should be much more stable than the pre cat sensor- but will still change a little, especially when revved.

wide band sensors, normally 5 wire are fixed voltage and alter the current for the signal.

 

[MetalMonkey]

It is either going to be a sensor or the cat.

The pre-cat sensor should be cycling quite quickly, the post-cat should be far more steady, but still moving slightly.
If the post-cat sensor is cycling rapidly like the pre-cat, then it is indicative that the cat isn't working.

Sensors operate between 0-1v, (0v=lean and 1v=rich) . It should usually be around 0.5v
1.2v seems high, and if it isn't fluctuating at all, it may suggest an issue with a sensor.

Given the sensor is probably a lot cheaper to replace than the cat, I know which one I'd try first!

 

[steveo3002]

ran it up from cold today , sensor 2 is 1.27v instantly from cold....sensor one takes 20-30 secs to wake up as i guess it should

 

[m_c]

Could be a wiring issue.
Check the live data voltage with the sensor disconnected, and measure the actual voltage at the connector (vehicle side)

 

[Chippie]

ran it up from cold today , sensor 2 is 1.27v instantly from cold....sensor one takes 20-30 secs to wake up as i guess it should

Does the reading change when you disconnect the sensor?
What is the reading with the engine off?

 

[steveo3002]

Does the reading change when you disconnect the sensor?
What is the reading with the engine off?

will report back later , mrs got it at work

 

[jsf55]

If you can check, try and see if it's the same sensor pre and post cat. You might be able too swap them and rule out it being a wiring issue. go easy if it's an old rusty system !

 

[steveo3002]

new post cat sensor on , codes cleared and light comes on after a few miles

sensor seems to flicker around 0.1v ish ....is that saying cat is bad or?

 

[Chippie]

new post cat sensor on , codes cleared and light comes on after a few miles

sensor seems to flicker around 0.1v ish ....is that saying cat is bad or?

In my opinion, no not necessarily….

Did you buy a genuine or pattern sensor?

 

[steveo3002]

bosch sensor

 

[tigdlo]

A long shot but you have disturbed the wiring and a partial short to ground could cause a low signal, did you visually inspect the wiring or take any multimeter readings while you were there?

 

[steveo3002]

i havent touched the wiring at all , theres only like 2-3" of loom then it goes inside so slim chance its got damaged

no meter readings taken , i was fairly sure it would fix it , i can have a look , what should i be checking

 

[Chippie]

bosch sensor

ok,
Ill illustrate my question……

Bosch make sensors, Bosch make sensors fitted by oem’s too.
The oem user may issue a particular spec on manufacture, when it comes to fitting a replacement the replacement may be made by a well known manufacturer of sensor, but the equipment it hangs off may not be tolerant and throw a code….hence why if it was a dealer part or one supplied by the likes of GSF or Eurocarparts….

 

[armalites]

The term OEM is often used as "The same as the dealer would provide" but the reality is it just means the part was made by the same people who made the original part. It doesn't mean it's the same part.

 

[steveo3002]

yes okay...bosch not from ford dealer ( i did enquire about genuine but never heard back)

pretty sure the old one was duff anyway with the reading stuck like that

 

[Chippie]

The reading of 0.1v, where is that being measured?
Is that the live data reading or are you measuring with a dvm on the output from the sensor?

 

[steveo3002]

The reading of 0.1v, where is that being measured?
Is that the live data reading or are you measuring with a dvm on the output from the sensor?

on the live data from the scanner