SORTING RULES - post-catalyst fuel trim (P2096 / P2097) and the faults around it, as written before any complaint was sorted
PulsCar analysis of NHTSA public data, October 2026. License: CC BY 4.0
Fingerprint of the rules below (date and checksum of the unchanged rules file): 2026-10-21T14:02:17Z  041522ff5a181b3e4f36ea16246a471972c895cba303b61d1245e901625877c6  codebook.md
Fingerprint of the addendum for the second batch: 2026-10-21T14:27:30Z  40365d105acbf4e94ce5838c099afeb2046e9bb324fe34722366ec05cd781693  codebook_addendum.md

# Sorting rules - post-catalyst fuel trim complaints (P2096 study; P2097 sorted with it, P2098 / P2099 sister codes)

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). Only a few dozen owners print the code itself, so the pool also takes the wording owners use for the same
fault. A complaint is in the pool if its narrative matches any of the following case-insensitive text searches (R2, R3
and R4 also need one of the context words CTX somewhere in the same narrative; R4 needs an oxygen-sensor word AND a
catalytic-converter word):

1. R1 - the code P2096 or P2097 (bank 1) 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?2\s?0\s?9\s?[67](?:\s?[-:]?\s?[0O]{2})?(?![0-9])`
   `\b(?:code|codes|dtc|dtcs|error|cel|coded|fault)\W{0,6}#?\s?[0O]?2?0?9[67]\b`
2. R1s - a sister code of bank 2 (P2098, P2099), same spellings:
   `(?<![A-Z0-9])P\s?[-.]?\s?[0O]?\s?2\s?0\s?9\s?[89](?:\s?[-:]?\s?[0O]{2})?(?![0-9])`
   `\b(?:code|codes|dtc|dtcs|error|cel|coded|fault)\W{0,6}#?\s?[0O]?2?0?9[89]\b`
3. R2 - the sensor behind the catalytic converter in owners' words (post-catalyst, downstream, rear, second oxygen sensor, bank 1 sensor 2):
   `post[\s-]?cat(?:alyst|alytic)?|downstream\s*(?:oxygen|o2|o\s?2|lambda|air[\s/-]?fuel|a/?f)|(?:rear|second|2nd|secondary|after[\s-]?cat\w*|bottom)\s*(?:oxygen|o2|o\s?2)\s*sensor|bank\s*[12]\W{0,3}sensor\s*2|\bb[12]\s?s\s?2\b|oxygen\s*sensor\s*(?:behind|after|downstream)`
4. R3 - fuel trim or a lean / rich condition in owners' words:
   `fuel\s*trims?|too\s+(?:lean|rich)|(?:lean|rich)\s+(?:condition|code|fault|bank|mixture)|running\s+(?:lean|rich)|system\s+(?:lean|rich)`
5. R4 - an oxygen / air-fuel sensor together with a catalytic converter:
   `o2\s*sensor|oxygen\s*sensor|\bo\s?2\b|lambda|air[\s/-]?fuel\s*(?:ratio\s*)?sensor|a/f\s*sensor`
   together with `catalytic|catalyst|\bcat\b`
CTX (context words for R2, R3, R4): `check\s+engine|engine\s+light|\bcel\b|\bmil\b|\bcode|\bdtc|service\s+engine|emission`

Background for the reader (not a sorting rule): P2096 and P2097 are "post-catalyst fuel trim system too lean / too rich,
bank 1". The engine computer adds or removes fuel based on the oxygen sensor in front of the catalytic converter (short and
long fuel trim); the sensor behind the converter is used as a check on that correction. When the correction that the rear
sensor demands runs out of range for long enough, the computer sets the code. Owners and shops say "rear O2 sensor",
"downstream sensor", "sensor 2", "post-cat sensor", "fuel trim", "running lean", "running rich".

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 an oxygen-sensor / fuel-trim event on THIS vehicle. Use N when:
    - the match is false (the words appear in a list of unrelated parts, "o2" or "lean" means something else, "cat" is an
      animal or a Caterpillar machine, "rich" is a person's name);
    - the narrative only lists the words among many other complaints, with no statement that this vehicle had an
      oxygen-sensor, fuel-trim, lean / rich code or related repair;
    - the words come from another vehicle, a news story or advice, not this vehicle;
    - the catalytic converter is the only part involved (code P0420 / P0430, converter replaced or stolen, converter
      recall) and no oxygen sensor, fuel-trim or lean / rich finding is stated (that is a different study);
    - an oxygen sensor is mentioned only as a part replaced during a repair for something clearly unrelated.
    NOT N (relevant) when, on this vehicle: P2096 / P2097 / P2098 / P2099 is named; OR a rear / downstream / post-catalyst
    oxygen sensor fault, code or replacement is described; OR any oxygen / air-fuel sensor fault, code or replacement is
    described together with a check-engine light or a drivability symptom; OR a fuel-trim, "system too lean" or "system
    too rich" code or a running-lean / running-rich diagnosis is described (P0171, P0172, P0174, P0175 included); OR the
    catalytic converter was replaced or blamed together with an oxygen-sensor or fuel-trim finding.

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

S - Rear sensor replaced. The sensor behind the catalytic converter (rear, downstream, post-catalyst, sensor 2) was named
    by a shop, dealer or mechanic as the cause, or was replaced.
Z - Oxygen sensor replaced, position not given. An oxygen / air-fuel sensor (front, upstream, "O2 sensor" without
    position, or "oxygen sensors" both) was named as the cause or replaced, and the complaint does not say it was the
    rear one.
C - Catalytic converter. The converter (or the converter with its pipe) was named as the cause or was replaced as the
    repair for this fault.
X - Exhaust leak. A leak in the exhaust manifold, flange, gasket, pipe, flex joint or converter joint (air getting into
    the exhaust ahead of or at the sensor) was named as the cause or repaired.
F - Fuel or air metering. Injector, fuel pump or pressure, fuel quality, MAF, MAP, throttle body, vacuum or intake leak,
    PCV, EVAP purge valve or another fuel / air-path part was named as the cause or replaced.
W - Wiring or power. The oxygen-sensor connector, wiring, harness, a ground, a fuse / relay, or the sensor heater
    circuit was named as the cause or repaired.
P - Software or campaign only. A reflash / recalibration, relearn procedure, recall, TSB, special-coverage or
    warranty-extension procedure was the only repair stated (no part named as cause).
O - Another cause. A repair outside the groups above was named as the cause (engine or head problem, oil burning,
    ignition / misfire parts, ECM replacement, exhaust or transmission part, a different system), or the code or symptom
    was traced to something else.
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 most specific named part in the order
W, X, F, S, Z, C. Among P, O, K, U: P before O before K before U. A cause the owner merely guesses at does not count
(an owner's "probably the O2 sensor" 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 while driving or at a stop the engine stalled, shut off, lost power, went into
    limp / reduced power, or hesitated / surged badly because of this fault.
inspection - the complaint states that the vehicle failed, or could not pass, an emissions test / inspection / smog
    check, or was refused registration, because of this fault or its code.
recall - the complaint states that a recall, special coverage, service campaign, extension, TSB 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,inspection,recall,returned,note`
category is one letter (N S Z C X F W P 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.


## Addendum - second search (added after the first 493 complaints were sorted, before the added ones were sorted)
A second, wider search of all 782,908 narratives looked for what the first search could not match. It found 943
candidates outside the first 493. The sorting rules above were not changed. 337 of them were added to the set and
sorted twice under the same rules:
- narratives that name a fuel-trim code or a lean / rich diagnosis written in other ways (P0171, P0172, P0174, P0175 with
  spellings such as "PO171"; "system too lean", "runs rich", "fuel trim" without a catalytic-converter word);
- narratives naming the sensor behind the converter in other words (downstream, post-cat, rear, sensor 2, "O2 censor #2", B1S2);
- narratives that print a code of the oxygen-sensor circuit family (P0136-P0141, P0156-P0161, P2270-P2273) with a check-engine light;
- narratives printing the number only ("2098", "2097") after a code word.
606 candidates were NOT added: 578 describe an oxygen sensor replaced or blamed with a check-engine light but name
neither the rear sensor, a fuel-trim code nor a catalytic converter (front-sensor events), and 28 are air-fuel-ratio-imbalance
codes (P219x, P2195-P2198, P2A00), which are injector or purge-valve events. They are outside what this page is about.


## Rules used to settle disagreements and the consistency check (written while settling; they refine the rules above, they do not change them)
R1 a part named by a shop/dealer/mechanic as the cause, or replaced by anyone, counts even if the fix did not hold (returned=yes). A code a dealer merely read out ("found P0137 sensor 2 circuit low") is a finding, not a cause: U if nothing was replaced or blamed.
R2 position: a rear/downstream/post-catalyst/sensor-2 sensor named or replaced (alone or with the front one) -> S; an oxygen sensor with no position, or the front one -> Z.
R3 several parts replaced or named together: most specific in the order W, X, F, S, Z, C. Parts replaced in separate visits for separate codes: the part replaced for the oxygen-sensor/fuel-trim fault decides; if never fixed and one part was named last by the shop, that part.
R4 owner's own guess (no shop, dealer or replacement) -> U; a catalytic-converter-only complaint (P0420) where the oxygen sensor is only the owner's guess -> N.
R5 an oxygen sensor replaced as part of a big repair for something unrelated (turbo, electrical, transmission) with no stated sensor/fuel-trim fault -> N.
R6 a dealer's SUGGESTION to clear codes is not a clearing: K needs an actual inspection with no fault found or an actual clearing; otherwise U.
R7 cause stated by the maker/dealer outside the groups (oil use, fuel dilution, engine overhaul, a later technician diagnosis) -> O; bad fuel quality -> F.
R8 flags: returned = the same light/code/symptom came back AFTER a repair or reset; a light that went away by itself and came back years later is not returned.
R9 motorcycles and heavy trucks are outside the study -> N.
R10 flags: recall = a recall/TSB/campaign/warranty extension is tied to this vehicle's event (performed, offered, missing, asked about); an owner's belief that "it is a known problem" is not. stall = stalled, died, lost power; "hesitates", "close to stalling", sputtering, misfire jerking are not.
R11 an owner-written advocacy text that lists codes in general (not this vehicle's events) -> N or U per the facts it states; an owner finding stated as theory ("I think injectors") is U (R4).
R12 the air-fuel-ratio-imbalance codes (P219A-P219F, P2195-P2198, P2A00) by themselves, with no statement of a fuel-trim fault, a lean/rich diagnosis, an oxygen-sensor finding or a fuel-trim code, are injector / valve-train events outside this page -> N.
Where both sortings agreed on the category, the consistency check applied R12 to every complaint whose only match was an air-fuel-imbalance code, and moved 66 of them to N; no other agreed row was changed.

## Pool history
First search: 493 complaints, sorted twice. Second search (completeness check of the whole file): 943 candidates outside the first set; 337 added under the unchanged rules (see the addendum) and sorted twice; 606 not added, for the reasons in the addendum. Total sorted: 830. Final: 703 relevant, 127 not relevant (N).