P0126: owner reports and bulletins

Powertrain (generic, all makes). We haven't written the explanation for this code yet. Below is what owners and manufacturers have filed with NHTSA.

Got P0126 on your scanner? Tell the mechanic your year, make, model and any symptoms and you'll get the likely cause and what to check first. Ask about this code.

What owners report to NHTSA

9 complaints filed with NHTSA mention P0126, 9 of them since 2015, across 4 different models. The component owners most often filed it under: engine.

Most-reported models

VehicleComplaintsModel years reported
MAZDA CX-562019, 2020, 2021, 2023

Manufacturer service bulletins that name P0126

24 technical service bulletins and manufacturer communications filed with NHTSA mention P0126, from 2 makes. A bulletin means the manufacturer has documented a known cause or a fix procedure for that model. Examples:

  • MAZDA CX-3 (2019, 2020, 2021, 2022), bulletin 11006981: Some customers may experience a check engine light ON with DTC P0126:00 (Thermostat stuck open) and one or more of the following symptoms: Engine takes longer to warm up. Engine temperature gauge fluctuates. Low coolant …
  • MAZDA CX-5 (2018, 2019, 2020, 2021), bulletin 11009664: Mazda Motor Corporation has decided to conduct a Special Service Program (SSP) to extend the warranty coverage for a specific repair of the fail-safe thermostat in the coolant control valve on certain CX-5, Mazda3, CX-3,…
  • MAZDA MAZDA3 (2019, 2020, 2021, 2022), bulletin 11011133: Some customers may experience a check engine light ON with DTC P0126:00 (Thermostat stuck open) and one or more of the following symptoms: Engine takes longer to warm up. Engine temperature gauge fluctuates. Low coolant …
  • MAZDA MAZDA6 (2018, 2019, 2020, 2021), bulletin 11003349: Some customers may experience a check engine light ON with DTC P0126:00 (Thermostat stuck open) and one or more of the following symptoms: Engine takes longer to warm up. Engine temperature gauge fluctuates. Low coolant …
  • MAZDA CX-30 (2020, 2021, 2019, 2018), bulletin 10190956: Some customers may experience the CHECK ENGINE light ON with DTC P0126:00 (Thermostat stuck open) and one or more of the following symptoms: The engine takes longer to warm up, The engine temperature gauge fluctuates, Th…
  • LAND ROVER RANGE ROVER (2010, 2011, 2012), bulletin 10214036: Technical Bulletin: Engine MIL Illuminated with DTC P0126-26/P0128-00 Stored

Sources: NHTSA Office of Defects Investigation complaint database and manufacturer communications file, snapshots September 2026. A complaint or bulletin that mentions a code is a report about that vehicle, not a statement that the code has one cause.