A place for more technical discussions. Please make sure you post in the correct section on the site, this way it keeps the site tidy AND ensures you get a more relevant answer.
AFAIK, anywhere between 0.5 ish and 0.9V means the oxygen is low enough, much below means that it is not working, or there is a leak drawing in air. IIRC, they read about 0.2V in fresh air.
Anyway, you are chasing the wrong problem, a non-working catalyst or faulty post-cat sensor won't reduce power.
A partly clogged catalyst will reduce the power.
If it suddenly breaks, go back to the last thing that you did before it broke and start looking there
I am not "chasing the wrong problem". I understand very well that the cat being blocked causes performance problems. I have experienced this first- hand when my rear cat had gone (fallen apart and rattling in the cat). It was blocking the exhaust and causing a severe lack of power.
However, this time, there is no rattle from the exhaust, so I was hoping that the O2 sensor readings might prove or disprove that there is a blockage in the cat. I appreciate that maybe this is not possible.
I am confused by what you are saying. You said 0.9 is fine, however you said the sensor reads 0.2 in fresh air? Surely then the post -cat sensor should therefore read closer to 0.2 than the pre-cat sensor, instead of miles above the pre-cat sensor?
Anyway, if the cat is blocked, how can I confirm this before buying a replacement? I am talking about the 1st cat on the manifold.
To determine if the engine and CAT are working correctly the O2 sensor is looking for no oxygen (or very little) left post CAT. No or very little oxygen indicates that the combustion was correct and that the CAT is working.
If the post CAT sensor looked like the pre-CAT sensor then the two traces would look the same and yours don’t.
I am not sure you can use the O2 sensors to determine if the CAT is blocked or not.
Google ‘how to tell if a CAT is blocked’ then look through the various methods. Or change it, they are not expensive on a Jimny.
By the way if the under car CAT is blocked, it’s most likely the first CAT which has disintegrated so you will need to replace that as well.
Robin
2020 blue SZ5 (one of the last to be registered in the UK)
Ex 2011 Blue Jimny SZ4
Northumberland Jimny Blog
Sorry just read the last bit of your post. The O2 sensors are pre and post the second CAT, they wouldn’t be able to tell anything about the manifold CAT.
The manifold CATs don’t block they disintegrate and it’s that which blocks the second CAT. If you take the manifold CAT off you might be able to see if it has disintegrated, if it has, it may have blocked the second CAT.
2020 blue SZ5 (one of the last to be registered in the UK)
Ex 2011 Blue Jimny SZ4
Northumberland Jimny Blog
I think the pre cat sensor is before the first cat? It looks like it is on the manifold? Or is that a different sensor?
I have ordered a back pressure tester, which I understand I can connect in place of the pre cat sensor and see if the pressure goes up too much when stabbing the throttle.
I have had the rear silencer collapse internally before
and this gave the symptoms you are getting
if you put your hand around an inch from end of exhaust on yours then compare the pressure
to another jimny you will feel the difference
or put an hook in a wire coat hanger push it into the exhaust rotate and pull out
Mine had all the hair like silencer packing come out on the hook
I tried flushing and cleaning my no 2 cat to no effect. It did not look so bad either but stole 25 horses or more.
I Removed cat no 1 all together as it do not give any problem during MOT here if not discovered..
My New cat no 2 cost 80 ish euro.
I did all welding and work myself though.
I had no fault codes as i remember, but have changed a sensor now and then.
Had a broken cat on my m13 Swift and there it gave a fault code for two years until i changed the cat.
