PULSCAR ANALYSIS - P0421 (WARM-UP CATALYST EFFICIENCY BELOW THRESHOLD, BANK 1) IN NHTSA'S PUBLIC FILES PulsCar analysis of NHTSA public data, October 2026. License: CC BY 4.0 Attribution: "PulsCar analysis of NHTSA public data, October 2026." Published alongside https://www.pulscar.io/blog/p0421-code ------------------------------------------------------------------------------ Prepared October 2026. Source data: NHTSA public flat files (complaints received 1 January 2015 to 21 September 2026, the latest received date in the files; manufacturer communications; recalls since 2010; investigations) and documents downloaded one at a time from static.nhtsa.gov. 1. Complaints (210) First search (76): every complaint whose narrative names P0421, P0422, P0431 or P0432 by any spelling we searched (a letter O for zero, a space, hyphen or period after the P, the suffix 00, or a bare 421 after "code", "DTC", "error", "CEL", "coded" or "fault"), or describes a warm-up, pre-, front or close-coupled catalyst in owners' words, or uses the words "catalyst (system) efficiency". Complaints that name only P0420 or P0430 and match only the generic words (90) were left to the P0420 page. Second search (134): an independent search of the same file for what the first one missed, run after the first batch was sorted. It found no complaint naming P0421, P0422, P0431 or P0432 that the first search had missed; it found complaints describing a catalyst code or an efficiency fault in other words (tiers A and B in sorting-rules.txt). 1,389 more complaints (tier C: a converter blamed or replaced with a warning light, no code number, no efficiency wording) were found and not sorted. found_by in the complaints file says which search found each complaint. Sorting: the sorting rules (sorting-rules.txt) were written, dated and frozen before any complaint was sorted; an addendum for the second batch was frozen before that batch was sorted. Every complaint was sorted twice, independently, by the same written rules, each time from the rules alone, without sight of the other sorting. One category (N not a catalyst fault on this vehicle; C converter named, replaced or quoted; S oxygen or A/F sensor; X exhaust leak or exhaust part; E engine-side cause; F software or a campaign procedure only; O another cause; K checked and cleared; U nothing stated) and four yes/no flags (stall, inspection, recall, returned) were recorded each time; both sortings are in the complaints file (sorting1_*, sorting2_*). Agreement over all 210: category 189 (90.0%); stall 208 (99.0%); inspection 210 (100%); recall 204 (97.1%); returned 203 (96.7%). Settling: every disagreement was read and settled by one of the written rules R1 to R12 (listed in sorting-rules.txt); the rule applied is in the rule_applied column. A consistency check then changed 4 agreed rows under the same rules (resolution says "changed in consistency pass"). Result: 191 relevant, 19 not relevant. Counts are counts of complaints, not cars. A complaint is evidence that an owner wrote to NHTSA; nothing here is a failure rate. 2. Documents (166 files, 119 distinct documents) First search: NHTSA's index of manufacturer communications, entries whose summary names P0421, P0422, P0431 or P0432, or names a warm-up, pre-, front or manifold catalyst efficiency fault: 83 files; 18 more catalyst-efficiency recalls and programs were added by hand after the first search (found_by "added by hand"). Second search: 49 more files found by the independent search. One document had no PDF on NHTSA (10190370). In addition, the 17 papers of Kia's Soul safety recall 19V120 were downloaded from NHTSA's recall file. Reading: 88 documents were read in full (71 representatives of families of revisions and copies, and the 17 Kia recall papers); 78 further files (revisions, copies and look-alikes of the representatives) were compared with the representative line by line. A few papers whose text did not read cleanly (image-only pages) were read from images of their pages. Every verbatim quotation in the article was checked against the text of the named document. Model years, engines, build dates and coverage terms are as printed in the documents, never taken from NHTSA's index, which expands years. Coverage status "as of September 2026" is the authors' computation from printed terms. duplicate_of marks exact copies (same text as another file); 47 of the 149 manufacturer-communication files are exact copies. 3. Owner-reported amounts (16 figures in 14 complaints) One written inclusion rule (amounts below, fixed before the figures were decided): a dollar figure is included when the owner states it as a price, charge, bill or quote for diagnosing, repairing or replacing a catalytic converter, an oxygen or air-fuel sensor or an exhaust leak, or a diagnostic fee for that catalyst-code event, on that vehicle. Excluded: mileage written as "NNk"; vehicle values, payments, trade-ins; other systems; totals that mix the catalyst with other work unless the owner attributes the figure to it; restatements; other owners' costs. Status (paid, billed, quoted) is from the owner's own words. The file lists every included figure. 4. Limits The complaint file holds the cases where an owner wrote to NHTSA, in the owner's words, with no verification; owners whose trouble ended are mostly not in it. The shares reported describe the complaints, not all cars. The document set holds what NHTSA's index lists; a maker paper that is not in the index is not in it.