P00B3

Radiator Coolant Temperature Sensor Circuit Low

P00B3 is a generic OBD-II powertrain diagnostic trouble code: Radiator Coolant Temperature Sensor Circuit Low. 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.

Code
P00B3
Group
Powertrain
System
Coolant
Severity
Warning (MIL on, possible limp mode)
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What P00B3 means

Code P00B3 is stored when the engine control module (ECM) detects that the signal voltage from the radiator coolant temperature sensor (RCTS) circuit has fallen below the calibrated minimum threshold. The RCTS is a negative-coefficient thermistor: at low temperatures its resistance is high and signal voltage is relatively high; as coolant temperature rises, resistance falls and voltage drops. A circuit-low fault therefore paradoxically suggests an excessively hot reading or, more commonly, a short to ground somewhere in the signal wire that collapses the voltage to near zero regardless of actual coolant temperature. With a near-zero signal voltage the ECM interprets the coolant as extremely hot — far above normal operating temperature — and may command the cooling fans to run at full speed continuously, reduce fuelling, or in some calibrations enter a protective reduced-power mode. Prompt diagnosis is important: if the ECM commands fans based on false data it may mask an actual overheat, or conversely drain the battery from continuous fan operation. The fault can also originate from a failed sensor whose internal resistance has dropped to near zero.

Common causes

Most-frequently reported root causes when P00B3 is logged.

  • 1
    Short to ground in the RCTS signal wire, collapsing signal voltage below the ECM's minimum threshold
  • 2
    Defective radiator coolant temperature sensor with internal resistance near zero (internal short), reporting an implausibly high temperature
  • 3
    Corroded or melted connector pins at the RCTS harness plug creating a low-resistance path to ground
  • 4
    Damaged wiring harness with a chafed signal wire contacting the chassis or engine block
  • 5
    Coolant leak saturating the connector body and creating a conductive short across the terminals
  • 6
    Blown sensor ground fuse or incorrect wiring modification causing voltage reference issues
  • 7
    ECM fault affecting the RCTS analogue input channel (rare, diagnose after ruling out external causes)

Symptoms drivers notice

Check Engine Light (MIL) illuminated, often accompanied by a possible reduced-power warning
Cooling fans running continuously at full speed even when the engine is cold, due to the ECM interpreting a false overheat condition
Engine may exhibit reduced power or enter a protective limp mode if the ECM treats the falsely extreme temperature as a real overheat
Temperature gauge may read pegged at maximum or behave erratically
Poor fuel economy and rough idle caused by incorrect ECM fuelling strategy based on false temperature data
Possible battery drain from sustained high-speed fan operation

How to diagnose P00B3

A typical diagnostic flow when this code is present.

  1. 1
    Retrieve all stored codes and freeze-frame data; note the RCTS voltage and temperature value logged at fault time — near 0 V or extreme high temperature confirms circuit-low condition
  2. 2
    Visually inspect the RCTS connector, wiring harness, and sensor body for coolant leaks, melted insulation, or corrosion that could create a short to ground
  3. 3
    Disconnect the RCTS connector; if the fault code changes to circuit-open (P00B4) after disconnection, the sensor itself is shorted internally — replace the sensor
  4. 4
    With the sensor disconnected and ignition on, measure signal wire voltage at the harness connector; near 0 V with the sensor unplugged confirms a short to ground in the wiring rather than the sensor
  5. 5
    Trace the signal wire from the sensor to the ECM, checking for chafe points against the chassis, exhaust, or sharp brackets
  6. 6
    Test sensor resistance at known coolant temperatures; resistance near zero at ambient temperature confirms an internally shorted sensor
  7. 7
    After repairs, clear codes, perform a full warm-up drive cycle, and verify that cooling fans cycle correctly and RCTS readings are plausible before returning the vehicle

Related powertrain codes

Frequently asked questions

Why are my cooling fans running non-stop with P00B3?

A circuit-low voltage makes the ECM interpret the coolant as extremely hot. As a protective measure, the ECM commands the cooling fans to maximum speed. Once the short to ground or faulty sensor is corrected and the code is cleared, the fans will return to normal temperature-based operation.

How do I tell if the sensor is shorted or the wiring is shorted?

Disconnect the sensor connector. If the signal voltage at the harness side rises to the 5 V reference level with the sensor unplugged, the short is inside the sensor — replace it. If the voltage stays near zero with the sensor disconnected, the fault is in the wiring between the sensor and the ECM.

Can P00B3 cause limp mode?

Some ECM calibrations will reduce engine power if they detect an apparent extreme coolant overtemperature condition. Since P00B3 can drive the RCTS reading to an extreme high-temperature value, limp mode is possible depending on the vehicle manufacturer's strategy — particularly on turbocharged engines where thermal management is critical.

Is P00B3 dangerous to ignore?

Yes. Continuous full-speed fan operation drains the battery and can mask a real overheating event. More critically, if the ECM's cooling strategy is disrupted by false temperature data, an actual overheat could go unnoticed until serious engine damage occurs. Diagnose and repair promptly.

Disabling P00B3 in software

RaceTune can permanently disable P00B3 — 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.

Permanent
The monitor is disabled in the ECU itself — not just cleared. It cannot return.
Tailored to your file
Each patch is matched to your specific software version — never a one-size-fits-all file.
Reversible
The original file is always preserved. Reflash the stock to return the ECU to factory state.

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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