Misfiring, but only irregularly

[Audax]

You can partially check that a lambda sensor is working with Tech-II, you watch the pattern it generates to see if it is reacting quickly enough and within the correct limits. Of course they can still show that they are OK when they are on the way out.

[mikeloadsasaabs]

I'm not even sure you are running rich.  If its cold and you are doing short journeys, the engine will be running rich.  It will be just like having the choke out.  You really need to check the plugs after a run.

They've never looked like this before, and we haven't changed our driving habits in the last couple of years. It sounds like it's worth a punt to change the lambda sensor. I'm wondering now if the misfire was caused by it running too rich (or too weak) under the circumstances prevailing at the time.

[mikeloadsasaabs]

I've just been investigating a local source of sensors, they seem to list two probes, a "regulating probe", Saab part number 4570909, and a "diagnostic probe", part number 4570917. Does one of them fit before the cat and the other one after?? And are these the right parts for my car (2004MY B235R)?

[sgould]

I think these are the right ones.

From EPC:

Article : Heated oxygen sensor
PartNo : 4570909
Usage : 2000-2010, B235
Unleaded petrol.
Front.
Order Quantity : 1



Article : Rear heated oxygen sensor
PartNo : 4570917
Usage : 2000-2010, B235
Unleaded petrol.
Rear.
Order Quantity : 1

[mikeloadsasaabs]

Looks good then, the downside is that they are selling at €240 for the pair.....will have to think about this!

[sgould]

I've not looked at them closely.  But they may have a Bosch part number on them.  But non-standard pattern/generic parts may cause problems.

I had a pair on my old car after the Tesco fuel contamination saga.  Consumption was 10% better.  So a long time to recover the cost on fuel saving alone.

[mikeloadsasaabs]

These were Bosch parts, so they should be OK.

But presumably, if it's running rich, it will damage the cat with a consequent need to replace?

[Audax]

The cat can stand a considerable amount of abuse long before it gives problems.

[harryO]

I would guess that the DI is failing. Not a bad idea to carry a spare and if the plug is pulled on Saab, they could become more expensive and difficult to obtain. As for the sensors, take a look at: http://www.aa1car.com/library/o2sensor.htm

[stevetreacy]

Hi,
You can test the lambda sensor with tech 2 or some other readers. The specific limits are in WIS under "closed loop".

steve

[mikeloadsasaabs]

I had a session set up with some diagnostic kit but it fell through, but I'm planning to kickstart it and will report back.