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Q: Cel on failing smog i get p0400 then after cleaning egr i get p0421, new air filter, cleaned intake rplaced hoses what else can i

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My sub has a p0400 Cel. I cleaned egr valve and replaced hoses after 80 mikes Cel came backbone cleaned 3gr again Cel off after 89 mike Cel back on and p0421 code. I repkaced hoses egr vacumn cleaned intake repkaced air filter ran sea foam thrust gas no white smoke came out. Do i have to replace every part from egr to cat converter. It went smog auto shop that sold it to me refused to smog it I got ripped off put 300 in it so far please help thank you Lorie

My car has 240000 miles.
My car has an automatic transmission.

If the only code at present is P0421, then the diagnostic can be focused on possible causes of just that code. Some possible causes of P0421 include: a faulty catalytic converter (most likely cause unless other codes are stored as well); a faulty oxygen sensor; faults in the circuit to and from the oxygen sensor; and a faulty powertrain control module, although that is not common (however the possibility of program updates to the PCM should be considered). To get this fully and finally resolved, the recommended service is a check engine light diagnostic and I encourage you to schedule that diagnostic. If you have further questions or concerns, do not hesitate to re-contact YourMechanic as we are always here to help you.

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