I recently had a problem, for the second time, with the catalytic converter on my SLK320. The first cat went at 15k miles (nearside cat broke up) and Mercedes replaced it under warranty. This cat then broke up (nearside again) after 8k miles. This time it was out of warranty and Mercedes refused any goodwill payment and also refused to say why not. Their suggestion was that I pay them £1287 to put new cats on. I don’t think so.
I decided to get an after-market cat (at a cost of £460), partly because I was unwilling to be ripped of by Mercedes and partly because it didn’t seem to me that the OEM parts were of very good quality. All seemed well for a few hundred miles until the Check Engine Light came on, and the code indicated that the nearside cat was of “insufficient efficiency”. After trying several independents (including GF) and being told that the only solution was to fit the correct part (“after-market cats are low quality” etc), I decided that there was a possibility that the problem was the Lambda sensor. So, at great expense, (£130 each sensor) I had the pre-cat and post-cat sensor replaced and the code cleared. This seems to have cured the problem.
This got me thinking about what had happened. I think the probable scenario is that the pre-cat nearside Lambda sensor became faulty very early in the life of the car. This caused the cat to overheat and break up. The CEL didn’t come on because the Merc cats are so over-engineered that they can clean up anything – for a time at least. Mercedes then replaced the cat without investigating the problem. They didn’t replace the sensor, so the new cat just broke up again in double quick time. When I then put a new (slightly lower efficiency) cat on the post-cat sensor rapidly began complaining because the cat didn’t have the capacity to clean the emissions up.
The moral of the story is that one should always check WHY the cat broke up. So why didn't Mercedes check this when the first one went. When the second one went in the same way why didn't they put 2 and 2 together? It seems obvious to me now, and I’m not a mechanic.
Although I can’t actually prove that my theory is correct, I’m pretty sure that this is what happened. The circumstantial evidence is very strong. So I’m inclined to take further action and complain again to Mercedes. The only thing is, I’m wondering if I’ll end up even worse off than before. They could just reject my claim again and then revoke my Mobilo guarantee because I’ve fitted a non-OEM part. What d’you reckon? Call it quits, or push the claim further.
I decided to get an after-market cat (at a cost of £460), partly because I was unwilling to be ripped of by Mercedes and partly because it didn’t seem to me that the OEM parts were of very good quality. All seemed well for a few hundred miles until the Check Engine Light came on, and the code indicated that the nearside cat was of “insufficient efficiency”. After trying several independents (including GF) and being told that the only solution was to fit the correct part (“after-market cats are low quality” etc), I decided that there was a possibility that the problem was the Lambda sensor. So, at great expense, (£130 each sensor) I had the pre-cat and post-cat sensor replaced and the code cleared. This seems to have cured the problem.
This got me thinking about what had happened. I think the probable scenario is that the pre-cat nearside Lambda sensor became faulty very early in the life of the car. This caused the cat to overheat and break up. The CEL didn’t come on because the Merc cats are so over-engineered that they can clean up anything – for a time at least. Mercedes then replaced the cat without investigating the problem. They didn’t replace the sensor, so the new cat just broke up again in double quick time. When I then put a new (slightly lower efficiency) cat on the post-cat sensor rapidly began complaining because the cat didn’t have the capacity to clean the emissions up.
The moral of the story is that one should always check WHY the cat broke up. So why didn't Mercedes check this when the first one went. When the second one went in the same way why didn't they put 2 and 2 together? It seems obvious to me now, and I’m not a mechanic.
Although I can’t actually prove that my theory is correct, I’m pretty sure that this is what happened. The circumstantial evidence is very strong. So I’m inclined to take further action and complain again to Mercedes. The only thing is, I’m wondering if I’ll end up even worse off than before. They could just reject my claim again and then revoke my Mobilo guarantee because I’ve fitted a non-OEM part. What d’you reckon? Call it quits, or push the claim further.