Here is my story:
At ~82k my 03 automatic protege got a p0421 code (warm-up cat efficiency bellow threshold). The car was running perfectly fine and getting up to 30mpg highway at 80+MPH.
This past november at 105k the car had to pass the CA smog test.. but from what I read on here taking it in with the p0421 code was an automatic fail.
I took the car to the dealer to have him look at the problem. I was assured multiple times that my first cat is bad and needs to be replaced ($1400 with labor.. HAHAH). The mechanic was certain that both of the O2 sensors are working fine.
I figured I would try the resistor/capacitor MIL eliminator trick. After wiring everything up (I soldered it very nicely so that the eliminator fit in the original insulation and pass visual), reseting the ECU and driving 200miles the CEL did not come on so I took the car in for smog. I was hoping that even with the first cat bad the second cat would be good enough to barely pass. I was shocked when the results came back and my car passed with flying colors. All of the values were over 10X lower than they could have been.
what gives? (other than the dealer trying to take my 1400..)
Is it possible that the first CAT really is bad? or is my second O2 sensor overly sensitive?
My car still runs great and I do not need to fix anything. I'm just curious as to why the P0421 came on in the first place.
Thank you all for posting so much valuable information on here. (I've been reading posts for years now.. but never posted)
At ~82k my 03 automatic protege got a p0421 code (warm-up cat efficiency bellow threshold). The car was running perfectly fine and getting up to 30mpg highway at 80+MPH.
This past november at 105k the car had to pass the CA smog test.. but from what I read on here taking it in with the p0421 code was an automatic fail.
I took the car to the dealer to have him look at the problem. I was assured multiple times that my first cat is bad and needs to be replaced ($1400 with labor.. HAHAH). The mechanic was certain that both of the O2 sensors are working fine.
I figured I would try the resistor/capacitor MIL eliminator trick. After wiring everything up (I soldered it very nicely so that the eliminator fit in the original insulation and pass visual), reseting the ECU and driving 200miles the CEL did not come on so I took the car in for smog. I was hoping that even with the first cat bad the second cat would be good enough to barely pass. I was shocked when the results came back and my car passed with flying colors. All of the values were over 10X lower than they could have been.
what gives? (other than the dealer trying to take my 1400..)
Is it possible that the first CAT really is bad? or is my second O2 sensor overly sensitive?
My car still runs great and I do not need to fix anything. I'm just curious as to why the P0421 came on in the first place.
Thank you all for posting so much valuable information on here. (I've been reading posts for years now.. but never posted)
Last edited:
