I have Mazda 5, 2014, about 40K miles There is error code: p0421 - warm up Catalytic converter efficiency below treshold (bank1)

Engine is not vibrating, so I assume this is not a problem with spark plugs or wires.

I tried one of those OBD2 devices to get some additional info:

downstream O2 sensor (V) jumps between 0 and 0.9 and back; it drops to 0 pretty much every time I release gas pedal (while driving on a freeeway),

upstream O2 sensor (Current, mA) was around 0.03 and at some point jumped to -40, then to -127, then got back to -0.1

Mode6 scan returned OK status for all data points except this one:

Catalyst Monitor Bank 1
Manufacturer Defined Test ID range — 
This parameter is an identifier for the test performed within the On-Board Diagnostic Monitor.,    ,✗  
Status: Test Fail  MID: 21 TID: 80 
MIN: 1.4999819 
VAL: 0.9999879 
MAX 255.9930000

Here is some data collected while driving on a freeeway, around 60mph enter image description here

There is a TSB pointing to similar problem but for other Mazda models, not Mazda 5, according to this TSB "This is caused by improper control logic of the PCM that may detect false error of the A/F sensor, resulting in false detection of P0421. To correct the problem, the control logic of the PCM has been changed"

Questions: What could cause this problem? (afaik, catalytic converter is still under federal warranty ( under 80K) - could it be it, or should I try to replace O2 sensors?

What is Catalyst Monitor Bank 1? (sorry for being ignorant, I tried to google it but I still don't understand, if this is a part of computer or one of the sensors or something else)

Thank you!

1 Answer 1


In very simple terms the catalytic converter is an exhaust component that converts "bad" gasses into "good" gasses.

The computer of the car monitors how well this functions by looking at oxygen sensors. Your car actually has two catalytic converters. The terminology goes as follows. When something is closer engine in the exhaust it is termed as upstream. When something is further away from the engine it is termed as downstream.

P0421 is a code for the downstream converter.

Simply take the car back to the dealer and they will take care of whatever it needs to fix the P0421. As you yourself stated federally they have a 8 year 80k mile waranty.

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