[Eddamoo]

I scanned my car last week as one on my TPMS modules is on the blink. While I was in there I also noticed a Pre Cat O2 sensor code - Signal to low in the event of coasting / overrun.

It wasn't causing an EML so dutifully cleared it. After 100 miles it's back...

The car has done 95k now and after researching have found that O2 sensors get very inaccurate as time goes on. This got me thinking, does anyone here actually change them proactively?

[bishbosh]

I had a similar fault code with the pre cat o2 sensor: something like it not returning to normal quickly enough on overrun. Interrogated it with ista and got a lovely live graph of it. I cleared the code and it came back so cleared it again.

This was a few months ago and only just realised from reading your post is that when I checked for faults last week the o2 fault hasn't returned!

I wouldn't bother proactively changing the o2 sensor but would if I had persistent faults with it. I can't remember the cost offhand but it wasn't extortionate.

[Eddamoo]

Thanks bishbosh yeah i think our codes are the same then - a slow Lambda returning to normal is the easiest way for the car to know that the sensor is 'Lazy'. I may have a go at graphing it like you say! Is yours a similar mileage?

Some great comparisons here: https://www.newtis.info/tisv2/a/en/f...ntrol/Ht2UIpgb

As its slow to react to the idle after overrun, its going to be slow acting to any other throttle application, causing a LEAN message to be sent to the ECU. This in turn will dump in more fuel, with the resultant soot. Not great for the DPF.

Thankfully our pre-cat O2 sensor is super easy to get to, so i'll do a bit of experimenting over the next few days:
1. Graph Lambda in its current state
2. Remove and clean the O2 and re-test - get it upto 600C to blast off the carbon (Like an Italian tuneup on a bench!).
3. Buy a new one and re-test again - £70 ain't bad if i get +1MPG out of it.

[bishbosh]

Mines at 107k miles so similar mileage.
Interesting reading, the graph in mine was like graph 2 in that link, the red line slow to drop off.

I don't think it's necessarily going to be slow to other inputs, AFAIK ista tested for that and it responded normally to increased engine speed, just not when the engine speed dropped.

[bishbosh]

@Eddamoo did you get further with your o2 sensor? The fault on mine came back car just shy of 110k miles. It's obviously fouling up, so thinking about buying a new sensor.

[Eddamoo]

Originally Posted by bishbosh

@Eddamoo did you get further with your o2 sensor? The fault on mine came back car just shy of 110k miles. It's obviously fouling up, so thinking about buying a new sensor.

Sorry mate unfortunately not - Ended up going on holiday (couldnt cancel without a large loss) so been holed up in quarantine and the car is on life-support (battery maintainer).

I scanned the car on my return and gave up trying to get an ISTA diagnosis as the coolant took forever to warm up, even with a fast idle. The coolant needs to be 85 before checking the Lambda reading unfortunately.

Ive got an NTK sensor ready to go on however - think it was only £65 from Autodoc, so will likely just swap it out. Ive got a 22mm O2 socket on order as the exhaust manifold is quite low in the bay, and wont leave much spanner room.

[bishbosh]

I've just bought a Bosch o2 sensor which I hope to get sometime this weekend. Haven't got the correct socket, will ponder how to remove it. According to tis it requires 50nm when tightening.

[Eddamoo]

Originally Posted by bishbosh

I've just bought a Bosch o2 sensor which I hope to get sometime this weekend. Haven't got the correct socket, will ponder how to remove it. According to tis it requires 50nm when tightening.

Unfortunately itll be closer to 100+nm when trying to undo.

[bishbosh]

Originally Posted by Eddamoo

Unfortunately itll be closer to 100+nm when trying to undo.

That's what I'm afraid of!

[Eddamoo]

Hey bishbosh a quick update. I didn't have enough time to mess around but the code came up for me again so just got on with changing my sensor. Was pretty easy, and could do it in <10 mins given what i learnt. The TIS is crap so ill provide some help.

• I did mine cold and the O2 sensor didnt need much at all to break free - probably 60-70NM and it definitely wasn't seized solid. I used a socket with an offset 1/2" and it worked well with 2 extension bars.
• Photo1: The Plug was the biggest pain in the ass for me as its right at the back of the engine bay, remove the wire clips in your way and the pressure sensor (T15 i think) for much easier access.
• Photo2: What i didnt realise at first was that the connector (Red) can slide out of the housing (slide guides in Blue). Pop a flat driver on the right side of the connector and pry it directly left (toward the positive battery terminal). This makes it LOADS easier to disconnect/reconnect
• Be careful when torquing with offset sockets if you have one. The moment is larger at certain angles.

[bishbosh]

Brill, thanks for that! Very good timing as I'm going to change the sensor tomorrow. 1/2 extension bar arrived today and the offset o2 socket arrived yesterday.

Cheers

[bishbosh]

Eddamoo Just changed the sensor, you're right the connector was a pain in the backside to remove. As you say unclipping the wire grommet and removing the pressure sensor helped. I also removed the electrical connector the pressure sensor to make a bit more room.

The old o2 sensor came off easily, new one in and fresh zip ties used to secure the wire. I reset the adaptations for the exhaust system and currently letting the car go to sleep before I give it a drive. Thanks again

[Eddamoo]

Awesome! Glad you cracked it mate