P0126 OBD-II: Insufficient Coolant Temperature for Stable Operation

OBD II Fault Code

• OBD II P0126

OBD-II Code P0126 is defined as a Insufficient Coolant Temperature For Stable operation

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.

P0126 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 P0126 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

P0126 Diagnostic Theory for Shops and Technicians

Here's how to diagnose the P0126 code:

1. 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.
2. Connect the scanner and select the most factory like data stream for the engine sensors. Start the vehicle (make sure the heater is off) and watch the Coolant Temperature values change.
3. If the Coolant Temperature readings will not go above the 160–170º F mark within fifteen minutes, the thermostat is the most likely culprit.
4. Be sure to verify that the engine coolant temperature readings agree with physical reality, so use a lazer pyrometer to measure the temperature of the engine, so you don't mistakenly blame the engine cooling system for a problem that is being caused by high resistance in the coolant temperature sensor, its connections or circuitry.

When doing the P0126 diagnosis, be sure to verify that either the electric or mechanical cooling fan is not stuck in the "On" position because this will cause the engine to run at a very reduced operating temperature. Also, be sure to verify that the Intake Air Temperature Sensor readings are within reason, meaning that they are not too hot or too cold in relation to the outside air temperature and under-hood air temperature. A good rule of thumb is that the Intake Air Temperature reading is usually about 100º F below the Coolant Temperature readings after warm up. When cold starting the vehicle, the readings should begin at nearly identical values.