Open circuit in the ECT sensor 6 signal wire, such as a broken wire or corroded terminal; Faulty ECT sensor 6 with an internal open circuit or excessive resistance; Poor connection at the sensor connector or at the ECM harness connector; High resistance in the sensor ground circuit shared with other sensors; Damaged or chafed wiring harness between the sensor and the ECM; ECM internal fault on the sensor input channel (rare but possible after other causes are ruled out)

DTC codes / P2Bxx · Hybrid propulsion / P2BB9

P2BB9Engine Coolant Temperature Sensor 6 Circuit High

Q: What does code P2BB9 mean?

Engine Coolant Temperature Sensor 6 Circuit High. P2BB9 indicates that the engine control module (ECM) has detected an abnormally high voltage on the signal circuit of engine coolant temperature (ECT) sensor 6. The ECM expects a voltage within a calibrated range that corresponds to the current coolant temperature, and a sustained high voltage typically means the circuit is open or the sensor's internal resistance has risen beyond specification.

P2BB9 indicates that the engine control module (ECM) has detected an abnormally high voltage on the signal circuit of engine coolant temperature (ECT) sensor 6. The ECM expects a voltage within a calibrated range that corresponds to the current coolant temperature, and a sustained high voltage typically means the circuit is open or the sensor's internal resistance has risen beyond specification.

SystemPowertrain

Code typeGeneric (all makes)

SeverityMedium

Fix difficultyIntermediate

AreaHybrid propulsion

Symptoms you may notice

Illumination of the check engine light, often with no other noticeable symptoms
Coolant temperature gauge reading colder than actual or staying at the bottom of the scale
Poor fuel economy or rich fuel mixture because the ECM substitutes a default coolant temperature
Rough idle or hesitation when the engine is cold, as cold-start enrichment is misapplied
Electric cooling fan running continuously or not engaging at the correct temperature
In some vehicles, a message on the driver information display indicating a temperature sensor fault

Common causes, most likely first

Open circuit in the ECT sensor 6 signal wire, such as a broken wire or corroded terminal
Faulty ECT sensor 6 with an internal open circuit or excessive resistance
Poor connection at the sensor connector or at the ECM harness connector
High resistance in the sensor ground circuit shared with other sensors
Damaged or chafed wiring harness between the sensor and the ECM
ECM internal fault on the sensor input channel (rare but possible after other causes are ruled out)

How to diagnose and fix it

Scan all modules and record freeze-frame data for P2BB9 to confirm the fault conditions and check for related codes
Inspect the ECT sensor 6 connector and wiring for corrosion, looseness, or physical damage, and repair as needed
Measure the resistance of ECT sensor 6 at known temperatures and compare with the manufacturer's specification chart
Back-probe the sensor signal circuit at the ECM and check voltage with the key on, engine off, looking for a value near 5 volts indicating an open circuit
Perform a voltage drop test on the sensor ground circuit to rule out high resistance in the ground path
If wiring and ground are good, substitute a known-good sensor or simulate the sensor signal with a resistor decade box to verify the ECM responds correctly
Clear the code and road-test the vehicle to confirm the repair, ensuring the monitor runs and the code does not return

Why a high voltage does not always mean a bad sensor

A high signal voltage on an ECT circuit almost always indicates an open circuit rather than a shorted sensor, so a broken wire or unplugged connector is more likely than a failed sensor.
Some vehicles use multiple coolant temperature sensors for different control functions, and sensor 6 may be a virtual or calculated value derived from other sensors, so consult the specific model's wiring diagram before condemning a physical part.
A poor ground connection at the sensor or ECM can elevate the signal voltage without any visible damage, and a voltage drop test is the only reliable way to find it.
After replacing a sensor or repairing wiring, the ECM may need to relearn the coolant temperature rationality, and some vehicles require a specific drive cycle before the code will clear permanently.
A stuck-open thermostat can cause the engine to run cold and set a high-voltage ECT code on some strategies, so verify actual coolant temperature with an infrared thermometer before assuming an electrical fault.

Can I keep driving with P2BB9?

The vehicle is usually safe to drive short distances because the ECM substitutes a default coolant temperature, but fuel economy and cold-start behavior will suffer. Avoid sustained high-speed or heavy-load driving until the fault is repaired, as the cooling fan strategy may be compromised and could lead to overheating.

Tools that help

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Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.