Methodology

Every number on a code page comes from public data published by the National Highway Traffic Safety Administration (NHTSA). Below are the files we use and the way we count.

Sources

• Owner complaints come from NHTSA's Office of Defects Investigation, which publishes every vehicle complaint it receives. We use the files for complaints received from 2010 onward.
• Carmakers must send NHTSA copies of their technical service bulletins and similar dealer communications. NHTSA publishes a summary of each one with the make, model and model years it covers.

How we count

• A complaint or bulletin is linked to a code when its text names the code, including common spellings such as "PO420" with a letter O.
• Each complaint is counted once per code, even when NHTSA lists it under several components.
• Bulletins are counted per make and bulletin number. For the "what manufacturers prescribe" chart, revisions of the same bulletin are counted once and only repair bulletins are included.
• Symptoms and fix types are counted by phrases in the text. One complaint or bulletin can fall into more than one group.

Limits

Complaints are owner reports that no technician has verified, and many problems are never reported to NHTSA. A make with more complaints is not necessarily less reliable; it may simply sell more vehicles or have a more vocal owner base. Bulletin counts reflect how manufacturers document fixes, which differs between companies. Use the figures to decide what to check first. They do not replace a diagnosis.

We do not copy complaint narratives or bulletin text. Bulletin descriptions on code pages are our own short summaries; the bulletin number lets a dealer or technician look up the full document.