P00B6 Radiator Coolant Temperature / Engine Coolant Temperature Correlation

P00B6 is an OBD-II diagnostic trouble code that indicates the engine control module (ECM/PCM) has detected an implausible relationship between the radiator coolant temperature sensor and the engine coolant temperature sensor.

In other words, the control module is receiving temperature information from two different points in the cooling system and the readings do not agree in the way the ECM expects.

This is an important distinction: P00B6 does not automatically mean that one coolant temperature sensor has failed. The problem can be electrical, sensor-related, mechanical, or caused by an actual abnormal coolant temperature condition.

Why Does the Vehicle Need Two Coolant Temperature Measurements?

The engine cooling system can have considerably different temperatures at different locations.

The engine coolant temperature sensor, usually referred to as the ECT sensor, monitors coolant temperature associated with the engine.

A radiator coolant temperature sensor, where fitted, provides information about coolant temperature at or around the radiator.

The ECM can use both measurements to understand how effectively the cooling system is transferring heat.

A simplified sequence looks like this:

Engine generates heat → coolant absorbs heat → thermostat controls coolant flow → radiator removes heat → cooled coolant returns toward the engine

The temperatures at these locations should change in a logical relationship as the engine warms up.

For example, immediately after a cold start, the engine and radiator coolant may both be relatively cool. As the engine reaches operating temperature and the thermostat opens, the relationship between the two temperatures should change in a predictable way.

P00B6 is set when that expected relationship becomes implausible.

What Does “Correlation” Mean in P00B6?

The word correlation is the key to understanding this DTC.

The ECM is not simply asking:

“Is the coolant temperature high?”

Instead, it is asking something closer to:

“Do these two coolant temperature measurements make sense together under the current operating conditions?”

If one sensor reports a temperature that is dramatically different from the other without a plausible reason, the ECM can determine that something is wrong.

The disagreement could be caused by an inaccurate sensor, an electrical problem, poor coolant circulation, a thermostat problem, or another cooling-system condition.

P00B6 Is Not Necessarily a Sensor Failure

This is where the code can easily be misunderstood.

A technician may see P00B6 and immediately replace the radiator coolant temperature sensor or ECT sensor.

That is not always the correct repair.

The sensors may actually be reporting a real temperature difference caused by the cooling system itself.

For example, if coolant is not circulating correctly, the temperature near the engine can become very different from the temperature near the radiator.

Possible causes therefore include both electrical and mechanical problems.

Common Causes of P00B6

Several faults can produce this code.

Faulty engine coolant temperature sensor

The ECT sensor may have an internal fault and report an incorrect temperature.

Faulty radiator coolant temperature sensor

If the vehicle is equipped with a separate radiator coolant temperature sensor, it can become inaccurate or intermittent.

Damaged wiring

An open circuit, short circuit, damaged insulation, or poor connection can cause an incorrect temperature signal.

Corroded or loose connectors

Coolant-temperature sensors are commonly exposed to heat, moisture, vibration and contamination. Poor terminal contact can alter the signal.

Thermostat malfunction

A thermostat stuck open or stuck closed can significantly change the expected temperature relationship.

A thermostat stuck open can prevent the engine from reaching normal operating temperature efficiently.

A thermostat stuck closed can restrict coolant flow toward the radiator and cause excessive engine temperature.

Low coolant level

Insufficient coolant can interfere with normal circulation and create unusual temperature differences.

Air trapped in the cooling system

An air pocket can prevent a temperature sensor from seeing the coolant temperature that the ECM expects.

This can be particularly important after cooling-system repairs or coolant replacement.

Restricted radiator

A radiator with internal restriction or poor external airflow may not remove heat normally.

Coolant circulation problem

A failing water pump, damaged impeller, restricted passage, or another circulation problem can produce abnormal temperature differences.

Cooling fan problem

A radiator fan that does not operate correctly can cause radiator temperature to behave differently from what the ECM expects, especially at low vehicle speeds.

Incorrect coolant or cooling-system service

An incorrectly filled or improperly bled system can create air pockets and abnormal temperature behavior.

ECM/PCM problem

A control-module fault is possible but should generally be considered only after the sensors, circuits, and cooling system have been properly tested.

Symptoms of P00B6

The symptoms depend heavily on the actual cause.

Possible symptoms include:

• Check Engine Light

• Engine running hotter than normal

• Engine running cooler than normal

• Cooling fan operating unexpectedly

• Cooling fan running continuously

• Temperature gauge behaving abnormally

• Poor fuel economy

• Rough running

• Poor cold-start behavior

• Difficult starting

• Reduced engine performance

• Increased emissions

• Overheating

• Heater performance problems

• Thermostat-related temperature fluctuations

In some cases, the driver may notice almost no symptoms other than the Check Engine Light.

That does not necessarily mean the fault is insignificant. A cooling-system problem can become much more serious if the underlying issue progresses.

A Very Useful Diagnostic Clue: Compare the Temperatures From a Cold Start

One of the most useful ways to approach P00B6 is to examine live data before starting the engine.

After the vehicle has been sitting long enough to become completely cold, the engine coolant temperature and radiator coolant temperature readings should generally be reasonable in relation to the surrounding conditions.

They do not necessarily have to be identical.

However, a major unexplained difference is suspicious.

For example, if the engine has been sitting overnight and one sensor reports a temperature close to ambient while another reports an extremely high or extremely low temperature, the electrical circuit or sensor deserves attention.

Watch What Happens as the Engine Warms Up

The next step is to observe both temperature readings during warm-up.

The important thing is not simply the final temperature.

The way the temperatures change can provide valuable information.

A technician can monitor:

• ECT temperature

• Radiator coolant temperature

• Engine RPM

• Vehicle speed

• Cooling fan command

• Thermostat behavior where available

• Engine load

• Coolant temperature changes over time

The temperature relationship should make sense for the vehicle's cooling-system design.

A sudden unrealistic temperature jump can suggest an electrical or sensor problem.

A gradual but abnormal temperature relationship may point more toward coolant circulation, thermostat operation, radiator performance, or another physical cooling-system issue.

Thermostat Problems Can Be Misleading

A thermostat problem is particularly important because it can create a temperature relationship that looks like a sensor problem.

If the thermostat remains open when it should restrict flow during warm-up, coolant may circulate through the radiator too early.

The engine can then warm more slowly than expected.

If the thermostat remains closed when it should open, the engine side can become excessively hot while the radiator side remains comparatively cool.

Therefore, replacing a temperature sensor without checking thermostat operation can miss the real cause of P00B6.

Low Coolant Can Also Produce Strange Readings

The coolant level should be checked when the engine is completely cold.

If coolant is low, the problem should not simply be solved by adding coolant and clearing the code.

The reason for the low level should be investigated.

Possible sources include:

• Radiator leaks

• Hose leaks

• Water pump leakage

• Thermostat housing leakage

• Expansion tank leakage

• Radiator cap problems

• Heater-core leaks

• Internal engine coolant loss

An air-filled area around a sensor can also cause its temperature reading to behave differently from the actual coolant temperature.

Water Pump Problems Should Not Be Overlooked

The water pump is responsible for maintaining coolant circulation.

If the pump cannot move coolant properly, the temperature distribution throughout the cooling system can become abnormal.

Possible reasons include:

• Worn pump

• Damaged impeller

• Loose or damaged drive system

• Internal pump failure

• Restricted coolant passages

A vehicle can therefore have two perfectly functional temperature sensors and still set P00B6 because the actual coolant temperatures are behaving abnormally.

Cooling Fan Operation Matters Too

The radiator fan affects how quickly heat is removed from the coolant.

If the fan does not operate when required, radiator temperature can rise abnormally.

Possible fan-related causes include:

• Failed fan motor

• Fan relay

• Fuse

• Wiring problem

• Fan-control module

• ECM command problem

• Temperature input problem

Conversely, a fan that runs continuously may indicate a sensor or control problem.

Fan behavior should therefore be checked as part of the overall cooling-system diagnosis rather than treated as an unrelated issue.

Electrical Diagnosis of P00B6

If the temperature readings look electrically implausible, the sensor circuits should be tested.

Depending on the vehicle, the circuit can contain:

• Sensor reference voltage

• Signal wire

• Sensor ground or return

• Connector terminals

• ECM input

Many coolant temperature sensors are thermistors, often using an NTC characteristic in which resistance changes with temperature.

However, the exact circuit design varies by manufacturer.

For this reason, there is no universal resistance or voltage value that should be applied to every P00B6 diagnosis.

The correct procedure is to use the vehicle's wiring diagram and manufacturer specifications.

Inspect the Wiring Carefully

Temperature sensors are often located in areas exposed to significant heat and vibration.

Inspect for:

• Melted insulation

• Broken wires

• Chafing

• Corrosion

• Loose terminals

• Water contamination

• Coolant contamination

• Poor previous repairs

• Connector damage

A sensor can test correctly on the workbench while its wiring fails when the engine moves or reaches operating temperature.

This is especially important when the code is intermittent.

Use a Wiggle Test for Intermittent Faults

If the temperature signal occasionally jumps to an unrealistic value, a technician can monitor the live data while carefully moving the relevant wiring harness and connectors.

If the temperature reading suddenly changes without a corresponding physical temperature change, there may be a wiring or terminal problem.

This should be performed carefully and according to the manufacturer's diagnostic procedure.

Do Not Confuse P00B6 With Actual Overheating

P00B6 can occur because the temperature information is incorrect, but the vehicle can also have a genuine cooling-system temperature problem.

These are two different situations.

If the engine is actually overheating, the priority is protecting the engine.

Warning signs include:

• Temperature gauge entering the red zone

• Overheating warning

• Steam

• Coolant boiling or escaping

• Strong coolant smell

• Sudden coolant loss

• Cooling fan operating abnormally

• Engine knocking or severe running problems

If actual overheating is occurring, continuing to drive can cause severe engine damage.

How a Technician Should Diagnose P00B6

A good diagnostic process should separate the electrical problem from the mechanical cooling-system problem.

Start with the complete scan and freeze-frame data.

Then:

Check the coolant level and inspect for leaks.

The cooling system should be examined before assuming the sensors are defective.

Compare the temperature sensors with the engine cold.

Look for implausible differences before startup.

Monitor both temperature signals during warm-up.

The relationship between them is often more informative than one individual reading.

Inspect the sensors and connectors.

Look for corrosion, coolant contamination, loose terminals and damaged wiring.

Test the sensor circuits.

Check the appropriate reference, signal, return, continuity and short circuits according to the wiring diagram.

Evaluate thermostat operation.

Determine whether coolant begins circulating through the radiator at the expected point for that vehicle.

Check water-pump and coolant circulation.

Look for signs of poor circulation or overheating.

Check radiator and fan operation.

A blocked radiator or malfunctioning cooling fan can alter the temperature relationship.

Verify the repair with live data.

After the fault is corrected, the temperature readings should behave plausibly through cold start, warm-up and normal operating conditions.

Can P00B6 Cause Engine Damage?

The code itself does not damage the engine.

The underlying cooling-system fault can, however.

If P00B6 is caused by a sensor circuit problem and the engine is maintaining normal temperature, the immediate mechanical risk may be relatively low.

But if the code is caused by:

• Low coolant

• Thermostat failure

• Water-pump failure

• Radiator restriction

• Cooling-fan failure

• Poor coolant circulation

then continued operation can potentially lead to overheating.

That is why the actual engine temperature should be considered more important than the presence or absence of the Check Engine Light alone.

Can You Drive With P00B6?

There is no single answer for every vehicle.

If the vehicle has normal coolant level, normal temperature, no overheating warning and normal operating behavior, it may be possible to drive it to a repair facility.

However, if the temperature gauge is rising unusually, the engine is overheating, coolant is disappearing, or steam is present, do not continue driving.

A cooling-system problem can progress rapidly, particularly under heavy load, high ambient temperature or extended highway driving.

What P00B6 Does Not Automatically Mean

P00B6 does not automatically mean:

• The radiator is bad

• The ECT sensor is bad

• The radiator temperature sensor is bad

• The thermostat must be replaced

• The water pump has failed

• The radiator fan is defective

• The engine is overheating

• The ECM is faulty

The code tells you that the ECM has detected an unexpected relationship between coolant temperature signals.

Diagnosis determines why that relationship exists.

The Best Way to Repair P00B6

The correct repair depends on what testing reveals.

If the sensor is inaccurate, replace the appropriate sensor.

If a connector is corroded, repair the connector and terminals.

If a wire is damaged, repair the harness correctly.

If the thermostat is malfunctioning, replace the thermostat.

If coolant circulation is inadequate, investigate the water pump and cooling passages.

If the radiator or fan is responsible, repair the relevant cooling-system component.

After the repair, clear the DTC and verify that the two temperature signals behave normally under the conditions that originally triggered the fault.

Final Takeaway

P00B6 means the ECM has detected an implausible correlation between radiator coolant temperature and engine coolant temperature.

The fault can be caused by an inaccurate sensor or electrical circuit, but it can also be the result of a real cooling-system problem such as a faulty thermostat, low coolant, trapped air, poor coolant circulation, radiator restriction, or cooling-fan malfunction.

The most informative diagnostic step is often to watch both coolant temperature readings from a cold start through normal warm-up. This can reveal whether the problem is an electrical signal that does not make sense or a genuine temperature difference created by the cooling system.

Most importantly, do not replace a coolant temperature sensor solely because P00B6 is stored. The sensors, wiring, coolant level, thermostat, circulation, radiator and fan should be evaluated as one system.