SORTING RULES - P0421 STUDY (warm-up catalyst complaints)
PulsCar analysis of NHTSA public data, October 2026. License: CC BY 4.0

Reference code of the sorting rules below (written before any complaint was sorted):
2026-10-14T08:26:21Z
de76b415b9b5c9e03a0a033f1ece87b91bba53db8fc3508de1ea597839677353  (the sorting rules)

# Sorting rules - warm-up catalyst complaints (P0421 study)

Written before any complaint in the set was sorted.

## Pool (how the complaints were found)
NHTSA complaints received 2015-01-01 to 2026-10-04 (782,908 narratives; 2026-10-04 is the latest received date in the
flat files). A complaint is in the pool if its narrative matches any of the following case-insensitive text searches:

1. R1 - the code P0421 by any spelling (letter O for zero, a space, hyphen or period after the P, suffix 00), or a bare number after a code word:
   `(?<![A-Z0-9])P\s?[-.]?\s?[0O]\s?4\s?2\s?1(?:\s?[-:]?\s?[0O]{2})?(?![0-9])`
   `\b(?:code|codes|dtc|dtcs|error|cel|coded|fault)\W{0,6}#?\s?[0O]?421\b`
2. R2 - the companion codes P0422, P0431, P0432 (main catalyst bank 1, warm-up catalyst bank 2, main catalyst bank 2) by the same spellings:
   `(?<![A-Z0-9])P\s?[-.]?\s?[0O]\s?4\s?(?:2\s?2|3\s?[12])(?:\s?[-:]?\s?[0O]{2})?(?![0-9])`
3. R3 - the warm-up, pre-, front or close-coupled catalyst in owners' words, or the words "catalyst (system) efficiency":
   `warm[\s-]*up\s+(?:three[\s-]*way\s+)?(?:cat|converter)|light[\s-]*off\s+cat|\bpre[\s-]*cat|close[\s-]*coupled\s+cat|\bcat\s+light[\s-]*off|front\s+cat(?:alyst|alytic)?\b|\bWU[\s-]*TWC|catalyst\s+(?:system\s+)?efficiency`
   Narratives that match only R3 through the generic words "catalyst (system) efficiency" AND name P0420 or P0430 (the title of those codes) are left to the P0420 page and are not in this pool.

Background for the reader (not a sorting rule): an engine has a catalytic converter that cleans the exhaust. P0420 is
"catalyst system efficiency below threshold, bank 1" in most makers' lists. P0421 is "warm-up catalyst efficiency below
threshold, bank 1": the computer judges a catalyst close to the engine (the warm-up or pre-catalyst) on its own,
usually soon after a cold start. P0422 is the main (underfloor) catalyst on bank 1; P0431 and P0432 are the same on
bank 2. Some makers use the numbers differently (a diesel's aftertreatment catalyst, a maker that labels its only
catalyst "warm-up"): sort what the owner says happened, not what the code table says.

For each complaint record exactly one CATEGORY and four yes/no FLAGS. You see an excerpt around the matching words
(and the whole narrative when nothing matched). Code what the excerpt supports; if it is cut off at a point that
matters, code what is stated.

## RELEVANCE
N - Not a catalyst-code / catalytic-converter-efficiency event on THIS vehicle. Use N when:
    - the match is false (the words appear in a list of unrelated parts, the code is for another system, a different meaning of the words);
    - the narrative only lists the words among many other complaints with no statement that this vehicle had a catalyst fault, warning, code, symptom or repair;
    - the words come from another vehicle, a news story or advice, not this vehicle;
    - the catalyst is mentioned only as a part that was checked or replaced during a repair for something clearly unrelated.
    NOT N (relevant) when, on this vehicle: P0421, P0422, P0431 or P0432 is named; OR a catalyst
    "efficiency/warm-up/pre-cat/front cat" fault, warning or repair is described (a shop, dealer or the owner's code
    reader saying the catalyst is below efficiency counts); OR a catalytic converter was blamed, replaced or priced because of a check-engine light with a catalyst code.

## CATEGORY (one letter) - what was found or done, on this vehicle

C - Converter. A catalytic converter (any catalyst: warm-up, pre-, main, manifold-integrated) was named by a shop, dealer or mechanic as the cause, was replaced, or was quoted/demanded for replacement.
S - Sensor. An oxygen / air-fuel-ratio sensor (upstream or downstream, "O2", "A/F") was named as the cause or replaced, and no converter was replaced.
X - Exhaust leak or exhaust part. A leak, loose or wrong clamp, cracked manifold or flex pipe, gasket or exhaust pipe repair was named as the cause or was done.
E - Engine-side cause. A misfire, ignition coil/plug, injector, fuel mixture, oil or coolant burning, vacuum/intake leak or other engine fault was named as what damaged or fooled the catalyst test and was repaired.
F - Software or campaign only. A reflash/recalibration/ECM update, recall, TSB, special-coverage or warranty-extension procedure was the only repair stated (no part named as cause).
O - Another cause. Something outside the above (a different system, a diesel particulate/SCR part, wiring) was named as the cause.
K - Checked and cleared. The shop or dealer inspected or tested and found no fault, or only cleared the code, with no repair; or the light went out by itself and nothing was done.
U - No diagnosis or repair stated. The event is reported but nothing was found or done yet, or the complaint does not say; also an owner's own guess, a forum, a code reader or scan with no shop/dealer/mechanic mentioned.

Precedence when more than one fits: use the FINAL repair the complaint reports as having fixed it (or, if it never
got fixed, the cause the shop named last). A part replaced that the owner says did not help does not count. If
several parts were replaced together and no single one is named, use the order X, E, S, C. Among F, O, K, U: F before O
before K before U. A cause the owner merely guesses at does not count (an owner's "probably the converter" is U unless a
shop or the dealer said it, or the part was replaced by someone).

## FLAGS (yes/no)

stall - the complaint states that the engine stalled, shut off, lost power, went into limp/reduced power, or hesitated/surged badly because of this fault.
inspect - the complaint states that the vehicle failed, could not take or was told it would fail an emissions test / state inspection / registration because of this fault.
recall - the complaint states that a recall, special coverage, service campaign, extension, TSB, emissions-warranty claim or "known issue" notice for this vehicle was performed, offered, missing, refused, asked about, or is blamed or credited for this event.
returned - the complaint states that after a repair or a reset the same light, code or symptom came back, or the repair did not fix it.

## OUTPUT

One CSV line per complaint, in the order given, no header line:
`odino,category,stall,inspect,recall,returned,note`
category is one letter (N C S X E F O K U); flags are yes or no (for N, all four flags are no);
note is at most 15 words, no commas, saying what decided the category.


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ADDENDUM FOR THE SECOND BATCH (written before the second batch was sorted)
Reference code:
2026-10-14T08:39:55Z
191a8ced620b3445cc43c4d26108f4268fa1b02c41f4f8bb08c3009e89517b2a  (addendum to the sorting rules)

# Addendum to the sorting rules - second search (written before the second batch was sorted)

The independent re-search of the complaint file found no complaint naming P0421, P0422, P0431 or P0432 that the first search had missed in any spelling. It found complaints that describe a catalyst fault in other words. Two groups are added to the pool and sorted exactly like the first batch:
- TIER A: wording such as "catalyst / converter efficiency", "below threshold", a mistyped catalyst code (P0242, P0402, P04320, P0042), or a recall for catalytic-converter efficiency;
- TIER B: "code for the catalytic converter", "bad cat code", "converter failed" with a check-engine light, no code number.
A third group (TIER C: a converter blamed or replaced with a warning light, no code number and no efficiency wording, 1,389 narratives) is NOT added: it cannot be told apart from any other catalytic-converter fault and belongs with the converter-replacement-cost page.
Clarifications C1-C4 for the second batch:
C1. A mistyped code that a reader cannot tie to a catalyst (P0242, P0042) is relevant only if the narrative itself says the code is for the catalytic converter / catalyst.
C2. SCR / NOx catalyst codes on diesels (P20EE, P2C7A) stay N (different system).
C3. "Catalyst efficiency recall" named only as background while the event is another system stays N.
C4. A converter replaced that the owner says did not stop the light is U unless a shop then named another cause (same as the precedence rule).

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RULES USED TO SETTLE DISAGREEMENTS AND IN THE CONSISTENCY CHECK
Each rule below was applied to complaints on which the two independent sortings differed (column rule_applied in the complaints file), or, for R1 and R7, to complaints on which both sortings agreed but the written rules required another category.

R1 a part that was replaced and, by the owner's own words, did not fix the problem does not count as the cause (the precedence rule above): category U unless a shop named another cause that held.
R2 a warning that limp mode WILL come is not a stall; stall = stalled, shut off, lost power, limp/reduced power stated as happening.
R3 the owner's own statement that a part failed, with no shop, dealer or mechanic naming it, is U.
R4 only the words, with no statement of a fault, warning, code, symptom or repair on this vehicle -> N.
R5 recall flag: yes when a recall/campaign/warranty is performed, offered, asked about, refused or wished for; a campaign named only as background while the event is another system -> N (not relevant).
R6 the code named is the evidence of relevance even when the owner mislabels what the code means (P0422 called "evap"): relevant, category U if nothing was found or done.
R7 SCR / NOx catalyst efficiency codes (diesel urea system, P20EE, P2C7A) are a different system from the warm-up catalyst -> N.
R8 a campaign / recall that is scheduled or awaiting parts, with nothing repaired, is U (F needs a repair or update actually stated); performed -> F.
R9 returned = the same light, code or symptom came back after a repair or reset; a new problem after an unrelated campaign repair is not 'returned'.
R10 a dealer or shop who looked at the code and said no action is needed: K (checked and cleared).
R11 when the shop names a computer reprogram as the next/last step and no part is named as the cause: F.
R12 a test that found the catalyst good, with the monitor blamed: K.

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POOL HISTORY
First search: 166 complaints matched the searches in the pool section. 90 of them were set aside because they name only P0420 or P0430 and match only the generic words "catalyst (system) efficiency" (they belong to the P0420 page); 76 remained.
Second search: an independent search of the whole complaint file found no complaint naming P0421, P0422, P0431 or P0432 that the first search had missed in any spelling. It found 1,523 complaints that describe a catalytic converter problem in other words: 39 with "efficiency" or "below threshold" wording or a mistyped catalyst code (tier A) and 95 with "code for the catalytic converter" wording (tier B) were added (134, sorted as batch 2); 1,389 with a converter blamed or replaced with a warning light but no code number and no efficiency wording (tier C) were not sorted.
Result: 210 complaints sorted twice; 191 relevant, 19 not relevant (N).