UK-POLOS.NET - THE VW Polo Forum

Hi

I have a 1.2 Petrol Polo and the engine management light came on a few days ago. It gave up code P0420 - Bank 1 - catalytic converter is operating low, chatting to a VW mechanic he suggested it's either the O2 sensor or the cat needs replacing. I figured as the sensor is less than £100 I'd try that first.

Looking online has been confusing - as the code says bank 1, is this sensor 1? Also if you can offer any advice on where they might be would be amazing as I've had so much conflicting information.

Thank you!

Check the fault code again, it should say Bank 1 Sensor 1 or Bank 1 Sensor 2.

As your engine is an in-line, ie 3 in a roll cylinders, it only has one bank of cylinders, V engines have 2 banks.

S1 or Sensor 1 is the precat sensor, and serves as a controller for the basic fuel "mixture" ie length of spray times, sensor 2 is the post cat sensor and is just monitoring the "treated" exhaust - ie has no control other than triggering the emissions indicator.

Edit:- ah, I've read your posting again, so it will be the pre cat sensor, ie Bank1 Sensor1 that might have aged and now not controlling the mixture correctly though unfortunately it will probably end up be the cat that has run out of life, it always makes sense to protect a new cat with a new pre cat sensor, so changing that first makes a lot of sense.

Another Edit:- this is not based on personal experience, but buying a cheap aftermarket Lambda probe has, for some people worked out to be quite expensive as some of these engine control systems are a bit fussy about the signals they receive, and so some people have ended up needing to buy a proper sensor from either VW Group or NTK which is what I think, was fitted to these engines/cars.

Third Edit:- changed "car" to "cat" - finger trouble!

Thanks for the help, I'll have a go at replacing the pre-cat sensor at the weekend first and see how that goes.

Bumping this thread, as I have the same error code on my Polo 6R (1.4 engine) :/

Just wondered if changing the pre-cat sensor ultimately solved your problem, or was it the actual cat that had failed?