Possible Causes:
* Faulty Thermostat: This is the most common culprit. A stuck closed thermostat prevents coolant from circulating properly, hindering the engine from warming up. Replacement is usually necessary.
* Low Coolant Level: Check your coolant level in the overflow reservoir. Low coolant prevents efficient heat transfer. If low, refill with the correct 50/50 mix of coolant and distilled water. Address any leaks if coolant is consistently low.
* Coolant Leaks: Inspect all hoses, clamps, radiator, and the water pump for leaks. Repair or replace any damaged components. Look for signs of rust or corrosion.
* Faulty Coolant Temperature Sensor (CTS): This sensor provides the engine computer with coolant temperature information. A faulty sensor might give inaccurate readings, causing the code. Replacement is typically the solution. There are usually two CTS, one for the gauge and one for the PCM (Powertrain Control Module). The code is more likely related to the PCM sensor.
* Worn Water Pump: A weak water pump may not circulate coolant effectively. Listen for unusual noises from the pump (whining or grinding). A visual inspection may reveal leaks or damage. Replacement might be necessary.
* Wiring Issues: Check the wiring harness connected to the CTS for any damage, breaks, or corrosion. Repair or replace damaged wiring.
* Air in the Cooling System: Air trapped in the cooling system can impede proper coolant flow. Bleeding the cooling system (often done by loosening a bleed valve on the engine block or radiator) can resolve this. Consult your owner's manual for the proper procedure.
Repair Steps:
1. Check Coolant Level: This is the easiest and first step. Add coolant if needed, but look for the source of the leak if you have to do this frequently.
2. Inspect Hoses and Clamps: Visually inspect all hoses and clamps for cracks, leaks, or looseness. Replace as needed.
3. Check the Thermostat: This is often the problem. There are several methods to test a thermostat (using a pot of boiling water is common). A better method is to let the truck run till the thermostat is supposed to open, remove the thermostat and test it to see if it's functioning. Replacing is often the best solution.
4. Check the Coolant Temperature Sensor (CTS): Testing requires a multimeter. Consult a repair manual or online resources for the proper testing procedure. Replacement is usually straightforward.
5. Inspect Water Pump: Look for leaks or unusual noises. If suspicious, replacement is best.
6. Bleed the Cooling System: If you've worked on the cooling system, bleeding the air out is essential.
Important Considerations:
* Repair Manual: A repair manual specific to your 1999 Ford Ranger is highly recommended. It provides detailed diagrams, procedures, and specifications.
* Safety First: Always allow the engine to cool completely before working on the cooling system. Coolant is extremely hot and can cause serious burns.
If you're not comfortable performing these repairs yourself, take your Ranger to a qualified mechanic. Diagnosing the exact cause of P0125 might require specialized tools and expertise.
