P0125

What P0125 means

The ECM uses ECT to decide when to switch from open-loop (fixed fuel map) to closed-loop (O2 sensor feedback). If the engine never warms up, it stays in open-loop and burns more fuel.

Symptoms

• Check engine light
• Poor fuel economy
• Cold heater output
• Low temp gauge reading

Likely causes (most common first)

1. Thermostat stuck open (most common — same fix as P0128)
2. Failed ECT sensor reading low
3. Low coolant

Can I keep driving?

Low — mostly emissions/economy

Typical repair cost

DIY fixes can start near $20 (parts only). Shop repairs for P0125 typically run up to $300 depending on the underlying cause and vehicle. Start with the most common cause — Thermostat stuck open (most common — same fix as P0128) — before throwing parts at it.

Symptoms that match P0125

If you got here from a search query, one of these might describe what your car is actually doing:

• Car Overheating — Stop, Diagnose, Don't Push It
• Fuel Economy Suddenly Dropped — Where to Look First

Related codes

Confirm P0125 yourself in 60 seconds

AXLY.pro is a free iPhone app that pairs with any Bluetooth OBD2 adapter to read P0125 and every other stored code, plus live engine data. No subscription.