Sluggish performance and O2 sensor readings

I know that there a few threads on this but I have some specific observations...

My Jimny has for a while felt sluggish, even more than usual, and fuel consumption is down as well. I can only tell by the amount of miles I get out of a tank, which is normally around 220 miles (with 215/75/15's) but I am down to about 180 miles. Any kind of gradient and I am struggling to get up it in the higher gears, again even more than normal.

Any way I have got a code reader that shows live data feeds from the sensors and here are my findings.

Whilst the car was on the drive warming up, the sensor behind the cat (not the manifold sensor) was stuck at 1.275 volts; it did not move. The manifold sensor was chopping up and down as I would expect as the engine goes from rich to lean.

I drove it around the block and recorded the sensor data and found that the cat sensor started to return sensible voltage readings, but occasionally it would bottom out to near zero volts at high revs, but generally it would be around .7 to .8 volts. My understanding is that the cat sensor should be the more stable one and it seems that way, other than locking on 1.275 volts and bottoming out occasionally.

So my question is, does this look like a blocked cat or a knackered sensor? I can't image that getting stuck at 1.275v is anything to do with the cat, so my instinct is to replace the cat sensor.

Any thoughts?

That sounds about right. The rear sensor does appear to be biased to around 1.3V and drops from there as it warms up. At high revs you will be running outside of closed loop control so the reading will also change.
What reading do you get from the front sensor when its struggling under load?

Rhinoman wrote: That sounds about right. The rear sensor does appear to be biased to around 1.3V and drops from there as it warms up. At high revs you will be running outside of closed loop control so the reading will also change.
What reading do you get from the front sensor when its struggling under load?

Thanks, that is useful.

I am out tonight and there are hills, so I will plugin the reader and take some samples and let you know.

Watching with interest I have just started getting the same problem when its cold still gutsy for a 1300 but warmed up and on a hill or pulling away from junctions Defo lack of power but seem to find a power spot at about quarter throttle.

The sensor at the back of the cat is telling you how well the cat is performing, 1.25 volts is to high should be around 1volt or just under, the sensor before the cat should vary between 0.1 to 0.8 volts, but the time it takes to vary from high to low is aleo important ,ckeck the readings with the engine up to operating temperature.

BigT wrote: The sensor at the back of the cat is telling you how well the cat is performing, 1.25 volts is to high should be around 1volt or just under

No its correct, I can tell you that from experience and the limit given in the FSM is 1.3V when cold.