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john aka eauto
john aka eauto, The Car Guy
Category: Mazda
Satisfied Customers: 16011
Experience:  ase tech,shop owner
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I am getting a P0305 Error code along with a P0421. The 421

Customer Question

I am getting a P0305 Error code along with a P0421. The 421 code has been showing since I had all the plugs and coil packs replaced and doesn't worry me much immediately. The P0305 and the rough idle / poor acceleration is new.
Submitted: 1 year ago.
Category: Mazda
Expert:  john aka eauto replied 1 year ago.

have you checked the converters for blockage?

Customer: replied 1 year ago.
No, it is suspected that the 421 code is because the gasket on the ERG was reused after the plenum was removed to change out the back row of plugs and, as I said above, I am not too concerned about that as it is an explained problem. those were changed as part of changing all 6 coil packs and plug due to misfires in the #1 and #3, which are noted as problematic due to the water flow off of the plenum runs right down onto those two.
The immediate problem is the P0305 code. If I understand correctly, the #5 cylinder is the center front one.
Expert:  john aka eauto replied 1 year ago.

is the egr gasket leaking? why no egr code? if the converter is clogged broken down it will cause all the issues you complain of....

Customer: replied 1 year ago.
When I took the van to have the code checked at AutoZone, they gave me that diagnosis since nothing else indicated the convertor being a problem. But, no, I haven't checked it further. How would I check it for blockage?
Expert:  john aka eauto replied 1 year ago.

po421 is a converter code. a back pressure gauge would be the easiest way..most exhaust shops can check

Customer: replied 1 year ago.
I am trying to get into the shop for them to check the pressure as you recommended. When I know more. I will respond again (if the problem is not corrected). Be back to you as soon as I know something.
Expert:  john aka eauto replied 1 year ago.