Diagnostics Case Study
Been told you need a new catalyst? Don’t be so sure – you could be spending good money without solving the fault…
Thousands of cars needlessly have new, inferior quality catalysts fitted every year..
We were approached by a customer for a price to fit a replacement catalytic converter to his car. The car has only covered 81000 miles and was 9 years old. Manufacturers use the very best quality materials in the original catalyst. They are under obligation to make a vehicle good for 100,000 miles with Emissions management to ensure the car stays clean.
It is actually quite rare for an Original Equipment Catalyst to fail unless the mileage is well over 100,000 miles. In fact 140-150k on original an catalyst is possible, and still pass the MOT with ease.
So straight away I am thinking that perhaps this guy doesn’t actually need a new Cat. So I ask him to bring the car for live diagnostics. When an MOT station tests your cars exhaust it only reports CO, HC and Lambda. What you really need for proper diagnostics is a 4 gas report. This will show CO, HC, O2 & CO2 – with these variables proper diagnosis can be made.
By setting up a number of custom variables and comparing outputs from various sensors it’s possible to judge if any component is failing. Very often components can be performing incorrectly but no Check Engine Light is lit.
When the car arrived we connected into the engine management system and after the car had warmed up it was easy to see that the Lambda sensor had failed. It had failed in such a way that the car thought the mixture was lean. So it added more fuel. The sensor still reports a lean condition, so it adds more fuel, and so on. Pretty soon the poor old catalyst has a real job on it’s hands as the mixture is being richened to the limit allowed by the ECU.
The customer was slightly surprised as the part was recently fitted by him, but it was a cheap aftermarket one. I wrote him a report, he got a free replacement part from the supplier, and heh presto with a working Lambda sensor the car soon returned to normal and passed the MOT with ease.
Total cost to him for my diagnostics £25. He was quoted for a new cat fitted by other garages from £500 for an aftermarket catalyst to £1200 for a genuine one. As you can imagine this was a customer who was very happy that I challenged the assertion he needed a “new cat”.
