Open or damaged wire in the radiator coolant temperature sensor signal circuit; Corroded, loose, or unseated electrical connector at the radiator coolant temperature sensor; Failed radiator coolant temperature sensor (internal open circuit or incorrect resistance); Poor ground connection for the sensor circuit, including engine or chassis ground points; Faulty engine control module (ECM) driver or internal circuit, though this is less common; Aftermarket or incorrect sensor installed that does not match the vehicle's resistance curve; Harness damage from heat, chafing, or rodent activity near the radiator

DTC codes / P00xx · Fuel and air metering and auxiliary emission controls / P00B4

P00B4Radiator Coolant Temperature Sensor Circuit High

Q: What does code P00B4 mean?

Radiator Coolant Temperature Sensor Circuit High. P00B4 means the engine control module (ECM) has detected an abnormally high voltage on the radiator coolant temperature sensor circuit, which usually indicates an open circuit, a poor connection, or a sensor that has failed in a way that raises signal voltage. The ECM expects the sensor to pull the reference voltage down as coolant temperature rises, so a high reading tells the computer the radiator coolant is colder than it actually is or that the circuit is broken.

P00B4 means the engine control module (ECM) has detected an abnormally high voltage on the radiator coolant temperature sensor circuit, which usually indicates an open circuit, a poor connection, or a sensor that has failed in a way that raises signal voltage. The ECM expects the sensor to pull the reference voltage down as coolant temperature rises, so a high reading tells the computer the radiator coolant is colder than it actually is or that the circuit is broken.

SystemPowertrain

Code typeGeneric (all makes)

SeverityMedium

Fix difficultyIntermediate

AreaFuel and air metering and auxiliary emission controls

Symptoms you may notice

Illuminated check engine light, often with no other noticeable symptoms
Cooling fan may run continuously or not engage at the correct temperature
Temperature gauge or dashboard coolant display reads colder than normal or stays at the bottom
Poor fuel economy or slightly rough running if the ECM substitutes a default temperature value
In some vehicles, a message on the driver information center about a temperature sensor fault
Overheating risk if the fan strategy is affected and the driver ignores the warning light

Common causes, most likely first

Open or damaged wire in the radiator coolant temperature sensor signal circuit
Corroded, loose, or unseated electrical connector at the radiator coolant temperature sensor
Failed radiator coolant temperature sensor (internal open circuit or incorrect resistance)
Poor ground connection for the sensor circuit, including engine or chassis ground points
Faulty engine control module (ECM) driver or internal circuit, though this is less common
Aftermarket or incorrect sensor installed that does not match the vehicle's resistance curve
Harness damage from heat, chafing, or rodent activity near the radiator

How to diagnose and fix it

Scan all modules and record freeze-frame data for P00B4 to confirm the fault is active and note the reported temperature
Inspect the radiator coolant temperature sensor connector and harness for corrosion, looseness, or obvious damage
Measure sensor resistance with a multimeter and compare it to the manufacturer's temperature-resistance chart
Back-probe the signal circuit at the ECM connector and check voltage with the key on, engine off, looking for a stuck-high reading
Perform a voltage drop test on the sensor ground circuit to rule out a poor ground
Repair or replace the faulty wiring, connector, or sensor as indicated by the tests, then clear the code and verify the temperature reading rises smoothly during a road test
If the circuit and sensor test good but the code returns, follow the manufacturer's diagnostic flow for ECM internal fault confirmation before replacing the ECM

Radiator sensor faults that fool good technicians

A radiator coolant temperature sensor is often a second, separate sensor from the engine coolant temperature sensor, so replacing the engine coolant sensor will not fix P00B4.
The ECM may default to a fixed coolant temperature when the signal is high, which can mask driveability symptoms and make the fault appear intermittent.
Corrosion inside the connector pins can create a high-resistance open circuit that only shows up when the engine bay is hot or wet, so wiggle-testing the harness is essential.
A poor ground shared with other sensors can set P00B4 alongside unrelated codes, so always check ground integrity before condemning the sensor.
Aftermarket sensors with the wrong resistance curve are a common cause of repeat high-voltage codes even when the original sensor was not defective.

Can I keep driving with P00B4?

You can usually drive a short distance to a repair shop, but avoid heavy loads, long trips, or stop-and-go traffic because the cooling fan may not activate correctly and the engine could overheat. If the temperature gauge climbs or the warning light flashes, stop safely and shut the engine off.

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.