Open circuit or high resistance in the sensor wiring or connector, such as corrosion, a bent pin or a loose terminal; Faulty radiator coolant temperature sensor with an internal open circuit; Sensor signal wire shorted to a voltage supply; Damaged, chafed or heat-affected wiring harness near the radiator or fan; Poor or missing sensor ground connection, which raises the signal voltage; Fault in the engine control unit's sensor input or supply circuit.

P00B4

Radiator Coolant Temperature Sensor Circuit High.

Generic
Powertrain
Generic code
Medium severity

How this code is built

P: SystemPowertrain
0: Defined bySAE, the same on every car
0: AreaFuel and air metering, auxiliary emission controls
B4: FaultThe specific fault in that area

What it means

P00B4 means the engine control unit has detected a signal voltage that is too high on the radiator coolant temperature sensor circuit. This sensor is separate from the usual engine coolant temperature sensor and monitors coolant at the radiator outlet or in the radiator circuit. On most sensors, which lower their resistance as they warm, a high voltage means the circuit is open or shorted to a voltage supply, so the unit sees an implausibly cold reading.

Symptoms

Engine management light on
Cooling fans running constantly or at an odd time, depending on the model
Temperature gauge reading oddly, or not behaving normally (model dependent)
Poor fuel economy if the control unit is using a default temperature value
Possible reduced performance or limp mode on some models

Possible causes

Open circuit or high resistance in the sensor wiring or connector, such as corrosion, a bent pin or a loose terminallikely
Faulty radiator coolant temperature sensor with an internal open circuitlikely
Sensor signal wire shorted to a voltage supplypossible
Damaged, chafed or heat-affected wiring harness near the radiator or fanpossible
Poor or missing sensor ground connection, which raises the signal voltagepossible
Fault in the engine control unit's sensor input or supply circuitrare

How to diagnose

Read all stored codes and freeze frame data, and note whether other coolant temperature or cooling fan codes are also present.
Check live data for the radiator coolant temperature and compare it with the engine coolant temperature once the engine is warm. A stuck cold or extreme reading points to a circuit fault.
Visually inspect the sensor, connector and harness for damage, corrosion, water ingress, chafing and loose pins.
With the sensor unplugged, check the reference voltage and ground at the connector against the manufacturer's data for that model.
Measure the sensor's resistance and compare it with the manufacturer's temperature and resistance table. An open circuit reading confirms an internal fault.
Check the signal wire for continuity to the control unit and for a short to voltage, with the ignition off and the control unit connector disconnected as the procedure requires.
Check the cooling system itself, including coolant level, flow and fan operation, so a real cooling problem is not overlooked.
If the wiring and sensor are good, check the control unit's connector and supply before suspecting the unit itself.

How to fix

Repair or replace damaged wiring, and clean or refit the connector terminals.
Replace the radiator coolant temperature sensor if its resistance is out of specification or open circuit, then refill and bleed the cooling system as required.
Repair any short to voltage in the signal wire.
Restore the sensor ground connection if it is poor or missing.
Replace or repair the control unit only after all other causes are ruled out, following the manufacturer's procedure.

Can I keep driving?

Medium severity

The car can usually be driven for a short time, but book it in soon. A faulty temperature reading may affect fan control and fuelling, and a real overheating risk could be missed, so stop if the temperature gauge rises or a warning appears.

Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.