1997 Ford Ranger P0125 Code: Insufficient Coolant Temperature for Closed Loop Fuel Control - Diagnosis & Fix

Hi everyone, I'm helping my uncle troubleshoot his 1997 Ford Ranger that's throwing DTC P0125 — 'Insufficient coolant temperature for closed loop fuel control'. The temperature gauge shows the engine is in the cold range most of the time, though it occasionally rises to the cold line. Here’s the freeze frame data: - Fuel System 1: Closed loop using O2 feedback - Fuel System 2: Not used - Calculated Load: 54.1% - Engine Coolant Temperature: 58°C - Bank 1 Short Term Fuel Trim: +3.1% - Bank 1 Long Term Fuel Trim: -2.3% - Bank 2 Long Term Fuel Trim: -3.9% - Engine RPM: 1800 RPM - Vehicle Speed: 16 km/h My initial thought is that the ECT (Engine Coolant Temperature) sensor isn’t faulty — the gauge does show some temperature rise, which suggests it's functioning. Since both the ECT sensor and ECT sender are typically on the same circuit, I don't believe both could fail simultaneously. I suspect a stuck-open thermostat is the root cause. However, even after driving the truck for extended periods in winter (40–50°F outside), the engine doesn’t reach normal operating temperature as indicated by the gauge. Is it possible that a stuck-open thermostat would prevent the engine from reaching proper operating temperature? I’d like to confirm this before recommending a repair. Also, is there any way to test the ECT sensor without a scan tool — for example, by immersing it in hot water and checking voltage output? Thanks for your insights! This will help me guide my uncle through the fix safely and effectively.