Use the first ten to fifteen minutes of warm-up behavior to decide whether slow temperature rise comes from a thermostat problem, low coolant, or false temperature reporting.
If the temperature gauge barely climbs, the heat stays weak, and the engine takes forever to settle into normal fueling, move into the cool-running branch first. That symptom architecture is different from a vehicle that is truly overheating in traffic or under load.
A thermostat stuck open is still the most common reason an engine takes too long to warm up. It keeps coolant circulating too early, so the engine sheds heat faster than it should during the first part of the drive.
If the cabin heat is decent, the upper hose behavior feels normal, or an infrared check says the engine is warm while the scan tool insists it is cold, stop treating the thermostat like the obvious winner. P011A, P2183, P2184, and P2185 exist specifically because temperature reporting itself can become implausible.
A low coolant level or recent cooling-system service can uncover one sensor, create uneven warm-up, or delay heater performance enough to look like either a thermostat problem or a bad sensor. That is why coolant level and service history belong near the front of the checklist.
First confirm coolant level and whether the heater output matches the reported temperature. Second watch warm-up trend from cold start instead of only the final gauge position. Third compare sensor readings with physical evidence. That order usually reveals whether you have a real slow-warmup problem or a false temperature story.