P0125 OBD-II: Insufficient Coolant Temperature for Closed Loop Fuel Control

What Does P0125 Code Mean?

OBD-II Code P0125 is defined as a Insufficient Coolant Temperature For Close Loop Fuel Control

What does this Mean?
In order for the Powertrain Control Module or PCM to effectively control the fuel and ignition systems while, at the same time, minimize the emissions output from a vehicle, the engine cooling system must reach a critical operating temperature. This temperature is typically between 160-170 degrees F and must be reached within 15 minutes after a 'stone cold' start. If the weather is extremely cold, say 10 or more degrees below zero, the coolant temperature must rise at least 70+ degrees from the 'stone cold' starting temperature. A 'stone cold' start is achieved when the vehicle is started after sitting with its engine off for at least 8 consecutive hours. If the coolant temperature stays below the 160-170 degree range or wanders above and below this level, the PCM can't rely on the exhaust oxygen feedback data it receives from the oxygen sensors in the exhaust system. When this occurs, the PCM must rely on a crude 'limp home' type of fuel and ignition control program. This raises the level of exhaust gas pollution to unacceptably high levels and will trigger the check engine light.

P0125 Symptoms

• Check Engine Light on
• Vehicle may not shift into the highest gear at freeway speed
• Decrease in fuel economy
• In unusual cases, there are no adverse conditions noticed by the driver

Common Problems That Trigger the P0125 Code

• Defective Engine Thermostat
• Defective Engine Coolant Temperature Sensor
• Defective Intake Air Temperature Sensor
• Defective Cooling System
• Low Engine Coolant
• Dirty Engine Coolant, causing incorrect Coolant Temperature Sensor readings
• Defective, always running Engine Cooling fan(s)

Common Misdiagnoses

• Engine Cooling Fan
• Internal Engine problem
• Oxygen Sensor problem
• Engine Coolant Temperature Sensor problem

Polluting Gases Expelled

• HCs (Hydrocarbons): Unburned droplets of raw fuel that smell, affect breathing, and contribute to smog
• CO (Carbon Monoxide): Partially burned fuel that is an odorless and deadly poisonous gas

P0125 Diagnostic Theory for Shops and Technicians

Here's how to diagnose the P0125 code:

• It is critically important to record the freeze frame data to determine which operational mode set the code. Pay close attention to the MPH, TPS, LOAD, RPM, and of course, the Engine Coolant Temperature and Intake Air Temperature. These values will help determine if the vehicle was being driven at freeway or slower, in town driving speeds.