Planet-9

Check Engine Codes 421 & 301

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Original source

2 captured repliesPartial discussion capture · Source accounts, not a confirmed diagnosis

THIS DISCUSSION / SOURCE-DERIVED OVERVIEW

Discussion summary

YOUR RESEARCHCheck Engine Codes 421 & 301

Leading reported evidence

Replace left cat due

Performed · outcome unconfirmed

No successful repair reported for this finding.

0
Reported fixes: 0
Unsuccessful attempts: 1
Discussions

Read evidence and context

Explicit resolution reports before identified causes, performed actions and suggestions. Within each level, matching component labels precede direct query mentions and inherited thread context; successful versus unsuccessful discussion counts, owner confirmations and independent discussion support determine order. This is not a probability of success.

Discussions that performed work without a confirmed outcome: 1

0 original-poster resolution reports. Counts can overlap; they are not unique people or a success rate.

0 discussions have direct query context; 0 report a fix in that context. Other evidence may inherit the discussion’s subject.

Performed · outcome unconfirmed
Other background intel: In May of last year the dealer replaced the left cat due to error code 431.
Original post Read captured discussion
Check Engine Codes 421 & 301

Frequently discussed work

Replace factory exhaust manifolds

No successful repair reported for this finding.

0
Reported fixes: 0
Unsuccessful attempts: 1
Discussions

Read evidence and context

Discussions that contain suggestions: 1

0 original-poster resolution reports. Counts can overlap; they are not unique people or a success rate.

0 discussions have direct query context; 0 report a fix in that context. Other evidence may inherit the discussion’s subject.

Suggestion
What should have been done was to replace the factory exhaust manifolds with the updated versions from Porsche and flash your
Original post Read captured discussion
Check Engine Codes 421 & 301

Read full discussion summary

Leading evidence: 'Replace left cat due' (no explicit attributable resolution; this is a reported action or suggestion). Across 1 matching discussions, The most frequently discussed action is 'Replace factory exhaust manifolds' (1 discussions). For that action, 0 discussions report a resolution and 0 report an unsuccessful attempt. No action has an explicit, attributable resolution report under the current rules. These are forum reports and suggestions, not a confirmed diagnosis.

Source reports, not a diagnosis or a success probability. Counts refer to discussions and can overlap.

Read captured replies

1 matching discussions · 2 captured posts examined · 1 partial discussions

Compare 4 other evidence results

Test there was no need

Performed · outcome unconfirmed

No successful repair reported for this finding.

0
Reported fixes: 0
Unsuccessful attempts: 1
Discussions

Read evidence and context

Discussions that performed work without a confirmed outcome: 1

0 original-poster resolution reports. Counts can overlap; they are not unique people or a success rate.

0 discussions have direct query context; 0 report a fix in that context. Other evidence may inherit the discussion’s subject.

Performed · outcome unconfirmed
Unless the coils or plugs tested bad there was no need to replace them unless you simply wanted to.
Original post Read captured discussion
Check Engine Codes 421 & 301

Replace factory exhaust manifolds

Suggestion or failed attempt

No successful repair reported for this finding.

0
Reported fixes: 0
Unsuccessful attempts: 1
Discussions

Read evidence and context

Discussions that contain suggestions: 1

0 original-poster resolution reports. Counts can overlap; they are not unique people or a success rate.

0 discussions have direct query context; 0 report a fix in that context. Other evidence may inherit the discussion’s subject.

Suggestion
What should have been done was to replace the factory exhaust manifolds with the updated versions from Porsche and flash your
Original post Read captured discussion
Check Engine Codes 421 & 301

Replace plugs and coils

Suggestion or failed attempt

No successful repair reported for this finding.

0
Reported fixes: 0
Unsuccessful attempts: 1
Discussions

Read evidence and context

Discussions that contain suggestions: 1

0 original-poster resolution reports. Counts can overlap; they are not unique people or a success rate.

0 discussions have direct query context; 0 report a fix in that context. Other evidence may inherit the discussion’s subject.

Suggestion
After conferring, the two techs suggested I replace the plugs and coils.
Original post Read captured discussion
Check Engine Codes 421 & 301

Run flashed

Suggestion or failed attempt

No successful repair reported for this finding.

0
Reported fixes: 0
Unsuccessful attempts: 1
Discussions

Read evidence and context

Discussions that contain suggestions: 1

0 original-poster resolution reports. Counts can overlap; they are not unique people or a success rate.

0 discussions have direct query context; 0 report a fix in that context. Other evidence may inherit the discussion’s subject.

Suggestion
Interestingly, the techs feel that running a flashed
Original post Read captured discussion
Check Engine Codes 421 & 301

Frequently discussed actions (5)

These are separate views of the same evidence. Frequency alone does not establish a fix, and these counts must not be added together.

Replace factory exhaust manifolds

No successful repair reported for this finding.

0
Reported fixes: 0
Unsuccessful attempts: 1
Discussions

Read evidence and context

Discussions that contain suggestions: 1

0 original-poster resolution reports. Counts can overlap; they are not unique people or a success rate.

0 discussions have direct query context; 0 report a fix in that context. Other evidence may inherit the discussion’s subject.

Suggestion
What should have been done was to replace the factory exhaust manifolds with the updated versions from Porsche and flash your
Original post Read captured discussion
Check Engine Codes 421 & 301

Replace left cat due

No successful repair reported for this finding.

0
Reported fixes: 0
Unsuccessful attempts: 1
Discussions

Read evidence and context

Discussions that performed work without a confirmed outcome: 1

0 original-poster resolution reports. Counts can overlap; they are not unique people or a success rate.

0 discussions have direct query context; 0 report a fix in that context. Other evidence may inherit the discussion’s subject.

Performed · outcome unconfirmed
Other background intel: In May of last year the dealer replaced the left cat due to error code 431.
Original post Read captured discussion
Check Engine Codes 421 & 301

Replace plugs and coils

No successful repair reported for this finding.

0
Reported fixes: 0
Unsuccessful attempts: 1
Discussions

Read evidence and context

Discussions that contain suggestions: 1

0 original-poster resolution reports. Counts can overlap; they are not unique people or a success rate.

0 discussions have direct query context; 0 report a fix in that context. Other evidence may inherit the discussion’s subject.

Suggestion
After conferring, the two techs suggested I replace the plugs and coils.
Original post Read captured discussion
Check Engine Codes 421 & 301

Run flashed

No successful repair reported for this finding.

0
Reported fixes: 0
Unsuccessful attempts: 1
Discussions

Read evidence and context

Discussions that contain suggestions: 1

0 original-poster resolution reports. Counts can overlap; they are not unique people or a success rate.

0 discussions have direct query context; 0 report a fix in that context. Other evidence may inherit the discussion’s subject.

Suggestion
Interestingly, the techs feel that running a flashed
Original post Read captured discussion
Check Engine Codes 421 & 301

Test there was no need

No successful repair reported for this finding.

0
Reported fixes: 0
Unsuccessful attempts: 1
Discussions

Read evidence and context

Discussions that performed work without a confirmed outcome: 1

0 original-poster resolution reports. Counts can overlap; they are not unique people or a success rate.

0 discussions have direct query context; 0 report a fix in that context. Other evidence may inherit the discussion’s subject.

Performed · outcome unconfirmed
Unless the coils or plugs tested bad there was no need to replace them unless you simply wanted to.
Original post Read captured discussion
Check Engine Codes 421 & 301

Coverage and how to read this overview

Counts describe captured discussions, not unique people or a diagnosis. Author identities are not reliably stored. Exact duplicate evidence is counted once across matching discussions; one discussion counts at most once per action/state. Multiple repairs reported together remain a combination. Suggestions and failed attempts do not establish a fix. Matches cover whole discussions and can include related faults, model variants and incomplete threads.

2 eligible posts · 0 unverified posts · 1 discussions match the subject in their title or opening post.

Planet-9: 1 discussions

0 evidence records excluded for query context · 0 excluded by outcome qualification · 0 adjusted by outcome qualification.

0 acknowledgements have no verified action link; they do not establish a repair result.

Generation: 20261027T031046Z · outcome-qualification-6. Deterministic extraction; no AI model inference. How we use sources

POST 1Original post

In May, I hit 5 years/60K and decided to buy my 06 Cayman S off lease. Co-indecently (or by design), while driving home from dinner last week, the CEL came on. There were no other symtoms or extream conditions. When my tuner shop connected in, we learned the following engine error codes: 421 (Efficiency..?) 301 (Misfire in Bank 1..?). After conferring, the two techs suggested I replace the plugs and coils. They only removed the CEL and it has stayed off since, awaiting my decision. Otherwise, no other symtoms/noises. Does anyone have any experience with these codes and their meaning? What may have caused it? After 60K the plugs are due. Interestingly, the techs feel that running a flashed ECU may have caused the ignition coils to retire early. Where is the best place to purchase replacements? Stay with OEM Bosch ? I am aware of updated coil packs. Are they the best bet? Other background intel: In May of last year the dealer replaced the left cat due to error code 431. Thanks for your thoughts, -Stevie

POST 2Original post

A search here will turn up what to do about the 421 and 431 cat efficiency codes. What should have been done was to replace the factory exhaust manifolds with the updated versions from Porsche and flash your ECU with the lastest Porsche flash. That would have fixed both of those codes. As for the misfire code, it's not uncommon, even in normal operation, to get some misfires, but also if you've ever tracked your car and ingested oil you will get misfires registered. Unless the coils or plugs tested bad there was no need to replace them unless you simply wanted to. I would recommend clearing the misfire code and see if it comes back and if it does, how often.

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