2007 1.6 TSē Engine Check Light (P0420) - Mazda3 Forums : The #1 Mazda 3 Forum
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post #1 of 1 (permalink) Old 09-07-2019, 01:04 PM Thread Starter
Go-Kart Driver
Join Date: Feb 2004
Posts: 90
Question 2007 1.6 TSē Engine Check Light (P0420)

Hello all,

I'm hoping someone can help me out here...

I've had the amber engine check light on for a while now. The OBDII code reads back as P0420 - Catalyst System Efficiency Below Threshold (Bank 1).

Every time I reset the code, it seems to come back after about 10-15 miles.

I have monitored the output from both O2/Lambda sensors. The first sensor on the exhaust manifold has an almost sinusoidal waveform. The post CAT sensor has a steady waveform when the engine is idle but almost follows the first sensor when driving (0.1V lower). This seems to indicate that the sensors are working but not sure if the CAT is faulty or blocked? Should the post CAT sensor waveform always be much lower than the first sensor?

I have also checked the short-term and long-term trims and CAT/coolant temperatures.

I've recently added a CAT cleaner into the fuel tank, but the light is still present after driving about 70 miles.

Does anyone have any other suggestions?

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