19/08/2026
The cat gets replaced. The car fails again. Because the cat was never the fault.
A pre-cat lambda sensor is the reason the mixture is right, and when it drifts, the ECU keeps trimming to a signal that's lying to it. The engine runs rich, the emissions climb, and the MOT tester writes down a number that points straight at a converter doing exactly what it was built to do. Fit a new cat to that car and you've bought an expensive way to fail the retest.
Here's how to check, and it takes minutes. Engine at operating temperature, hold 2,500 rpm, high-impedance multimeter only. You want the signal swinging the full range, lean to rich, hundreds of times a minute, sitting somewhere between 0.4V to 0.6V.
Slow or flat, and you've found your fault for a fraction of the price of a converter.