PULSCAR ANALYSIS - U0121 (LOST COMMUNICATION WITH THE ABS CONTROL MODULE) AND ABS, TRACTION AND STABILITY WARNINGS 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/u0121-code ------------------------------------------------------------------------------ Scope and dates. Prepared October 2026 from NHTSA's public flat files: owner complaints received from 1 January 2015 to 4 October 2026 (the latest received date in the files; 782,908 complaints) and the index of manufacturer communications, plus the maker papers themselves from static.nhtsa.gov. The NHTSA complaint file is public: anyone can download it from NHTSA and check any number here against it. 1. Complaints counted (3,224) 1,950 complaints whose narrative names U0121 by any spelling we searched, or has communication wording ("lost communication", "no communication", "CAN bus" and similar) together with ABS wording, or has an ABS, traction control or stability control warning lamp together with battery, voltage, alternator, ground or wiring wording; all sorted. 500 more: a random sample (fixed seed 121, drawn before any sorting) of the 14,832 narratives that matched only wording about an ABS module failure or only wording about a warning lamp. The other 14,332 were not sorted; their ODI numbers are in complaints-not-sorted.txt. And 774 more found by a second search of the whole file, run after the first batch was sorted, for narratives written in words the first search could not match (anti-lock written out, VSA, VSC, ESP, StabiliTrak and similar names, other codes such as U0415 and U0416, lamps named in a list). The second search was limited to 800 added complaints, so more complaints of this kind exist than are counted here. The exact search wording is in codebook.txt. found_by in the complaints file says which search found each complaint. Each complaint was sorted twice by the same written rules (codebook.txt), into one cause category and five yes/no questions (brakes, comm, volt, recall, returned). The yes/no answers are floors: when a narrative does not settle a question, the answer is no. Result: 3,147 relevant, 77 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. 39 complaints spell out U0121 (names_u0121 column); the rest describe the event in the owners' words, which is why the set is built on wording as well as on the code. 2. Documents (u0121-documents.csv) Maker papers (bulletins, campaigns, special coverage, service updates), recall filings and owner letters, and NHTSA investigation summaries, chosen from NHTSA's index by what the index summary says: papers that name U0121, that describe lost communication with the ABS, brake or stability module, or that tie an ABS, traction or stability warning to the module's power supply, ground, connector, wiring, voltage or software. Recall filings are those of the campaigns whose index text matched most closely; the other recall campaigns that matched were not opened. One chosen entry has no file on NHTSA and is listed as "no file". The list shows, for each paper, whether it names U0121 in its own text, which other codes it names, whether it mentions an ABS or brake control module together with communication, network, voltage, battery, ground or power-supply wording, its relevance (DIRECT, LISTED, RELATED, NOT RELEVANT) and the NHTSA file address. Exact copies are marked in duplicate_of. Vehicle years, engines, build dates and coverage terms are as printed in the papers, never taken from NHTSA's index, which expands years. Coverage status "as of September 2026" is the authors' computation from printed terms. 3. Owner-reported amounts (414 figures in 353 complaints) A dollar figure is included when the owner states it as a price, charge, bill, quote or estimate for diagnosing, repairing or replacing a part or doing a job tied to the ABS, traction or stability warning or lost-communication event on that vehicle (the ABS or brake control module or hydraulic unit, a wheel speed sensor, wiring, a connector, a ground, the battery, alternator or charging system when the owner ties it to the event, software reprogramming, or diagnosis). Excluded: mileage written as "NNk"; vehicle price, value, payments, trade-ins, buybacks, refund demands and settlement sums; repairs of other systems; totals that mix other major work; restatements; other owners' costs; rental-car and towing costs. Status comes from the owner's own words (paid, billed, quoted, not stated). 650 candidate figures were found and 236 were excluded, each with its reason in u0121-excluded-figures.csv. Figures are what owners wrote, not audited invoices. 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 sorted complaints, not all cars and not the whole complaint file. Not every matching narrative was sorted: 14,332 that matched only the module-failure wording or only the warning-lamp wording were left out of the sample (complaints-not-sorted.txt), and the second search was limited to 800 added complaints. The sorted set therefore understates how many complaints of this kind exist, and the 500 sampled complaints stand for 14,832. The share of complaints that mention power or communication depends on which search found the complaint (found_by column) and must not be read as a share of all ABS complaints. Only 39 complaints print U0121; the rest describe the same kind of event in words. A warning lamp in a narrative is not proof that the ABS module lost communication. The cause category is the least certain field; that is why the article reports the share of complaints where no cause was stated. The document set holds what NHTSA's index lists and what could be read; a maker paper that is not in the index is not in it, and some listed papers were not downloaded or could not be read.