P00B4
Radiator Coolant Temperature Sensor Circuit HighP00B4 is a generic OBD-II powertrain diagnostic trouble code: Radiator Coolant Temperature Sensor Circuit High. It is logged by the engine control unit when the coolant monitor detects that a specific fault threshold has been exceeded — typically resulting in the malfunction-indicator lamp (MIL / check-engine light) being illuminated.
What P00B4 means
DTC P00B4 is stored when the engine control module (ECM) detects that the voltage or resistance signal from the radiator coolant temperature (RCT) sensor circuit is above the expected operating range. The radiator coolant temperature sensor is a thermistor mounted in the radiator or along the coolant lines; as coolant temperature rises, its resistance normally drops, producing a predictable voltage signal. A "circuit high" condition means the ECM is seeing an abnormally high voltage — often caused by an open circuit, corroded connector, or a failed sensor that is no longer changing resistance with temperature. Because the RCT signal is used to command cooling fan operation and to fine-tune fuel trim and ignition timing, a stuck-high reading can mislead the ECM into thinking the coolant is extremely cold, resulting in over-rich fuelling, unstable idle, and potential overheating if the cooling fans are not commanded on at the correct time. The code is generic (SAE J1979) and applies to all OBD-II compliant vehicles that are fitted with a dedicated radiator coolant temperature sensor separate from the engine coolant temperature sensor.
Common causes
Most-frequently reported root causes when P00B4 is logged.
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1
Defective radiator coolant temperature sensor (open internal element, stuck resistance)
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2
Corroded, damaged, or backed-out connector pins at the sensor harness plug
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3
Open circuit or short to voltage in the sensor signal wire
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4
Damaged wiring harness (chafing, heat damage, rodent damage) between sensor and ECM
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5
Faulty engine control module (ECM/PCM) — rare but possible
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6
Coolant leak exposing sensor to air rather than coolant, skewing resistance reading
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7
High contact resistance from moisture ingress into the sensor connector
Symptoms drivers notice
How to diagnose P00B4
A typical diagnostic flow when this code is present.
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1
Retrieve all stored DTCs and freeze-frame data with a scan tool; note any related cooling system or fuel trim codes
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2
Visually inspect the radiator coolant temperature sensor and its harness connector for corrosion, damage, pushed-back pins, or signs of coolant leakage around the sensor body
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3
With the ignition on (engine off), measure the signal voltage at the sensor connector; a reading above 4.7 V typically indicates an open circuit in the sensor or wiring
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4
Disconnect the sensor and measure its resistance across the two terminals using a multimeter; compare the reading against the manufacturer's temperature-resistance chart — a reading of OL (open loop) confirms a failed sensor
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5
Inspect the wiring from the sensor to the ECM for continuity and absence of shorts to battery voltage; repair any open or shorted sections found
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6
Clear codes and perform a test drive through a full warm-up cycle; if P00B4 resets, replace the radiator coolant temperature sensor and retest
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7
If all wiring and sensor test within spec, check for ECM technical service bulletins (TSBs) before condemning the module
Related powertrain codes
- P008F — Engine Coolant Temperature/Fuel Temperature Correlation
- P00B1 — Radiator Coolant Temperature Sensor Circuit
- P00B2 — Radiator Coolant Temperature Sensor Circuit Range/Performance
- P00B3 — Radiator Coolant Temperature Sensor Circuit Low
- P00B5 — Radiator Coolant Temperature Sensor Circuit Intermittent/Erratic
- P00B6 — Radiator Coolant Temperature/Engine Coolant Temperature Correlation
Frequently asked questions
Is P00B4 the same as P0115 (Engine Coolant Temperature Sensor)?
No. P00B4 refers specifically to the radiator coolant temperature sensor, which monitors coolant temperature at the radiator itself. P0115 relates to the engine coolant temperature sensor near the engine block or cylinder head. Many modern vehicles have both sensors to monitor the difference between coolant entering and leaving the engine.
Can I drive safely with a P00B4 code active?
Short trips are generally tolerable, but the code should not be ignored. If the ECM misreads coolant temperature, cooling fans may not engage properly, creating a real risk of engine overheating. Monitor the temperature gauge closely and repair the fault promptly.
How much does it cost to fix P00B4?
A replacement radiator coolant temperature sensor typically costs $15–$60 for the part. Labour is usually 0.5–1 hour, bringing total shop costs to roughly $80–$200 depending on vehicle accessibility and local labour rates. A wiring repair can vary widely.
Will P00B4 clear itself after repair?
Once the root cause is fixed, the code will clear automatically after a complete drive cycle if the fault is no longer present. For faster confirmation, use a scan tool to clear the DTC and then verify it does not return after a warm-up cycle.
Disabling P00B4 in software
RaceTune can permanently disable P00B4 — and any other OBD-II diagnostic trouble code — on every ECU family we support. The monitor is disabled inside the ECU itself, so the fault stops being logged: the warning light stays off and the engine never enters limp mode for this code. The change is tied to your exact software version.
Software modifications affect emissions compliance and are not road-legal in many jurisdictions. RaceTune service files are intended for motorsport, off-road, and export use.
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