P0125: How to Diagnose & Fix — Insufficient Coolant Temperature for Closed Loop Fuel Control

The 2.0L runs a marginal cooling system that is intolerant of neglect, and cooling faults are a leading contributor to the head-gasket failures above. The water pump (timing-belt driven) leaks or seizes, thermostats stick shut, and the lower radiator hose (with an internal anti-collapse spring) can collapse under pump suction at speed, choking flow. The system is also notoriously hard to burp — trapped air causes intermittent overheating even after repairs.

The integrated plastic-housing thermostat on the 3.6L Pentastar commonly fails stuck open or opens too early, so the engine never fully reaches operating temperature. The ECM sets P0128 (coolant temp below thermostat regulating temperature), and the driver gets weak cabin heat, sluggish warm-up, lower fuel economy, and increased oil contamination. It is one of the most frequently searched check-engine complaints on these engines and is an inexpensive, DIY-friendly fix.

Later 3rd-gen and 4th-gen 4Runners use a wide-band Air-Fuel Ratio (A/F) sensor as the upstream (Bank 1 Sensor 1) oxygen sensor. With age and heat cycling the sensor's internal heater or element degrades, setting codes such as P0125 (insufficient coolant temp for closed loop / A-F feedback), P1130 (A-F sensor range/performance), or P0135 (heater circuit). Symptoms include poor fuel economy, hesitation, and a check engine light. It's a very common high-mileage sensor replacement and a frequent search topic.

EcoSport owners report thermostats sticking open, leaving the engine running below its target operating temperature. The signature is a P0128 'coolant thermostat below regulating temperature' code together with weak cabin heat and worse fuel economy, because the ECU holds a richer warm-up strategy while the engine never reaches closed-loop temperature. What makes this awkward on the EcoSport is that owners on the 2018+ forum have found the vehicle carries TWO thermostats, so replacing the obvious one and reusing the other leaves the code coming back — and several owners report P0128 persisting through repeat coolant temperature sensor replacements because the sensor was never the fault. Coolant seepage from the plastic thermostat housing and its gasket is the closely related failure, showing up as slow, unexplained coolant level loss rather than a puddle. Both engines are affected; on the 1.0L the consequences are much worse because of its small coolant volume, so treat any coolant loss on a three-cylinder car as urgent.

The 2.3L Duratec-equipped Ranger is known for coolant leaks from the thermostat housing assembly and related plastic cooling components. Owners report seepage or sudden cracking near the housing, leading to coolant smell, low coolant level, and occasional overheating if ignored. The problem is common enough in owner forums and repair databases to be considered a known age-related weakness on these trucks.