Decat Exhaust

[paulc]

I take it, if the cats and secondary lambda sensors are removed then the check engine light will remain on?

[Kaz-kzukNA1]

If this is a general question, then removing the CAT alone won’t directly trigger the CEL but can do so indirectly.

ECU looks for the open/short circuit of the O2 sensor heater circuit AND monitors the sensor output voltage.
So, even just one of two or four O2 sensors showed open/short heater circuit or the sensor voltage out of the window for two consecutive drive cycle or detected twice, then CEL triggered.

So, yes, disconnecting the secondary lambda sensor would trigger the CEL immediately because of open heater circuit.

Removing the CAT won’t directly trigger the CEL but because the output voltage of the secondary/downstream sensor would be out of the window, it will trigger the CEL as a result.


However, if this question is related to your NSX, then this is not the case.

I don’t know the full spec of your NSX but based on the photo and contents of other threads, most likely, your NSX is NA1, MT, non-DBW so 91-94 model but don’t know whether it’s UK or JDM spec.

If above is true, then you only have 2 x primary/upper-stream O2 sensor on your NSX.

If JDM, there was a CAT temperature sensor with silver braided wire coming out from the CAT body.

This was required by law in Japan but from late ‘90s, no longer required so even removed from later spec NSX.

Disconnecting this temperature sensor won’t trigger the CEL or any lights in the dash.

May be you were referring to this temperature sensor as the secondary O2 one???


As a side note....

I don’t know the MOT rule in your area and it may change in 2024 so please take that into consideration.

While you ‘may’ fail MOT for the emission test if the engine not healthy, removing the CAT itself won’t fail the MOT if your NSX first used before 01/Aug/92 unless the tester got confused and told you that the CAT should be there as it was fitted from the factory.


Kaz

[paulc]

Hi Kaz, as always many thanks for your detailed explanation, really appreciated. Yes it the NA1, MT, non-DBW 91 model, JDM spec.

[wixer]

Your car is the same age as mine (1991) so pre dates any necessity for CATS in the UK. I've never ran mine with CATS until this year & it never failed an MOT, as the test is a visual check. The only reason I've fitted some free flowing CATS from Pride, is to reduce noise as I've got a Tubi exhaust fitted. They didn't really make any difference to the noise than the Pride resonated test pipes I had previously fitted.

As Kaz says, you shouldn't have the secondary lambda sensors, so nothing to worry about if you remove your CATS.