[stargatesheila]

I have a statement from Porsche GB that says Faulty Lambda Sensors cannot cause
damage to a Catalyst, but rather a failed
Catalyst can give a false indication that
Lambda Sensors are faulty.

I am being told a load of rubbish or is it true ?

[Hometune]

The purpose of the lambda sensor in front of the catalyst (cat) is to tell the engine computer (ECU) how much oxygen is in the exhaust. This determines the mixture which should be 14.7 parts air to 1 part fuel at all times.
If the mixture becomes weak, the sensor will detect the excess oxygen, tell the computer and it will increase the fuel to achieve 14.7 to 1.
If the mixture becomes too rich, the sensor will tell the computer to send less fuel.
If this sensor fails, the computer will not have a value to work from. So, if you have a rich mixture and excess fuel is being sent down the exhaust, it will irrepairably damage the 'cat'. A faulty airflow sensor or coolant temperature sensor can cause a very rich mixture amongst others.

A failed catalyst usually shows up as fault code P0420 on a diagnostic. If the front sensors are showing normal readings which switch from 0.1-0.8 volts at once a second and the second sensors, after the 'cat', are also switching the same then this is a sure sign the catalyst has failed. The second sensors should remain stable around the mid point of 0.5 volts or so.

A blocked 'cat' would cause the front sensor not to switch as a lack of flow of the exhaust gases would be detected. This could be interpreted as a failed sensor if a technician was not up to speed on diagnostics.

[stargatesheila]

Thanks "Hometune" I was very sure faulty Lambda Sensors could damage the Catalyst, but Porsche GB insist not (their exact final comments on my problem )
" Lambda Sensors not known to cause a Catalyst to fail. However a defective Catalytic converter can cause a false indication of a sensor failure etc "
Six months ago engine warning light came on and car was tested and faulty Lambda Sensors replaced under Warranty
Now 1500 miles later same problem, but a £1500.00 job. They said this is a separate event but if Cat had been found faulty 6 months ago they would have re-considered (total Mileage 26,000) They say 2 new Lambda Sensors needed again but this time not under Warranty but £350.00 must be paid for them. I am concerned that this is collateral damage and further tests should have been done 6 months ago, and in any case surely 26,000 miles is
not normal for Catalyst failure. No goodwill even forthcoming..thanks

[Hometune]

They have said themselves that a failed cat can give false lambda readings. Strikes me that this is what has happened here.
A 'cat' is usually lifed at 50,000 miles. Lambda sensors should last the same and in a lot of cases for 3 times that.
I would strongly advise you to contact Trading Standards through Consumer Direct. Often a call from them has the desired effect.
Also, if the garage is signed up to the latest Good Garage Scheme you can ask the RAC who police this, to take up the complaint on your behalf. You don't need to be a member.

[stargatesheila]

Hometune thanks again , one thing I forgot was . Porsche GB say new Lambda Sensors need the 1500 miles to adjust so that is why the engine light would have not come on until now. Is that correct ? , they add Lambda Sensors require a period of time (1500 miles in this case) to adjust to a particular engines operating characteristics (their exact words)

[Hometune]

Afraid its all more nonsense :eek:
Lambda sensors work instantly thats why they have heaters in them, to ensure that from cold, they are operating at their best.
The engine computer normally has a re-learn cycle after some components are replaced but this is a normal 20 minute drive and thats it.

This all sounds like the old garage trade versus single woman syndrome to me. No offence intended.

[stargatesheila]

Thanks once again , its given me a lot to go on , actually its Porsche GB technical dept who it appears have given all this crazy information to the writer of the letter which is from the customer care dept of their extended warranty dept. I was sure their complete reasoning and the 1500 miles and 6 months for the lambda sensors to adjust was a complete
nonsense, all the help very much appreciated

[stargatesheila]

Just a word of grateful thanks especially to "Hometune". After a complete refusal Porsche have paid back all the costs involved ,totalling £1377.24p.It seems after all the silly comments they put in writing, like it taking 6 months for the Lambda Sensors to adjust etc , their Insurance company took about 6 weeks to get them to respond to what they had said , because it was all a complete load of rubbish..Very much appreciate your input