P0420 Alarm But Not The Cat!

[petemagsati]

Hi all

P0420's seem to be a common post, and most of the answers say "yes, it's probably your cat". Well that's what Mr. Toyota told me when I got my diagnosis done. I didn't trust him for two reasons:

(a) I had asked them to reset the alarm, and it was on as soon as I got back in the car. When I asked them they said they did reset it and it came on straight away. This is not possible as the drive cycle before the car self diagnoses can take a couple of weeks depending on how you drive. Just to check I reset by unhooking the Battery and it took a few days to come on again.

(B) They told me to replace both the cat and the front lambda sensor. It's either one or the other, so I suspected even they weren't sure. After questioning this they said " It's definitely the cat, but we always replace the o2 sensor at the same time. Price for both including fitting: €1300.

Well, I just got my NCT done (Irish equivalent of the MOT), and the emission results came back as 0.00% CO and 0 ppm HC at low idle. I take this as meaning the cat is working perfectly. I would strongly recommend that anyone who has this alarm get's an emissions test done, because my impression is that garages will just throw parts at it at your expense.

Just ordered the lambda sensor for €200, hope it does the job. Will post again when my light goes off!