When will the car fail the MOT?

If a car is fitted with a decat Downpipe, it will not pass the MOT test. This is because the catalytic converter is visibly missing, which the inspector will notice as there is no catalytic converter present. You can have the second lambda sensor deactivated by a chip tuner to drive without any faults, but the car will still fail the MOT without a catalytic converter.

A Downpipe fitted with a 100-cell high-flow catalytic converter has 100 fewer cells than a 200-cell catalytic converter. This means that the flow rate – and therefore the airflow – is even better than with a 200-cell catalytic converter. However, problems with emissions can occur more frequently with a 100-cell catalytic converter. Fortunately, the four-gas emission test is no longer compulsory for petrol cars manufactured after 2012, which means that cars with a 100-cell catalytic converter can still pass the MOT. Nevertheless, the engine fault warning light must not be on.

How can I get my car through the MOT?

There are various methods used in practice. For example, you can fit a Downpipe with a 200-cell high-flow catalytic converter, which will result in MOT approval in the Netherlands. However, it is important to note that the 200-cell catalytic converter allows for significantly more airflow than the original catalytic converter. The problem with this is that you may still get an engine warning light (not always) because the second lambda sensor detects that the catalytic converter is not working. The catalytic converter is actually working, but the second lambda sensor does not detect this due to the much higher airflow. To clear this engine warning light, you can visit a chip tuner who can disable the second lambda sensor. Disabling the second lambda sensor has no further adverse effects on power, safety, and so on. This is purely a check to see whether the catalytic converter is functioning.

Another method people use is to drive around with a ‘decat Downpipe’ all year round and then replace it with the original Downpipe – which, of course, does have a catalytic converter fitted – during the MOT test. However, to drive with a decat Downpipe without any problems, you must ensure that the second lambda sensor has been deactivated by a chip tuner.

Are there any other tricks?

The well-known lambda spacer / lambda fix / Cel Fix / Cellfix is an extension of the lambda connection, sometimes containing a small catalytic converter. The idea is to fit this spacer between the lambda sensor and the nut on the exhaust, so that the car doesn’t display a fault light when a decat Downpipe or high-flow cat Downpipe (in some cases) is fitted. However, in my experience, this does not work in 9 out of 10 cases. I have tested it on various cars using a lambda spacer with a small catalytic converter / without a small catalytic converter / with a bend / without a bend, and it actually never works. We do not wish to mislead anyone and therefore do not sell this product.

This blog covers pre-2019 petrol cars (NON OPF).
Please note that this is for guidance only; we accept no liability.

Written on 23-22-2023 by Juul, JT-Products.