P0187

Fuel Temperature Sensor B Circuit Low Input

P0187 is a generic OBD-II powertrain diagnostic trouble code: Fuel Temperature Sensor B Circuit Low Input. It is logged by the engine control unit when the powertrain monitor detects that a specific fault threshold has been exceeded — typically resulting in the malfunction-indicator lamp (MIL / check-engine light) being illuminated.

Code
P0187
Group
Powertrain
System
Powertrain
Severity
Warning (MIL on)
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What P0187 means

P0187 is stored when the PCM determines that the voltage signal from fuel temperature sensor B has dropped below the minimum calibrated threshold — in practice, a voltage near or at 0 V on the sensor signal wire. Because the fuel temperature sensor is a negative temperature coefficient (NTC) thermistor, its resistance decreases as fuel temperature rises, which produces a lower voltage signal at higher temperatures. A signal voltage that stays permanently near zero therefore appears to the PCM as an impossibly high fuel temperature reading. The most common electrical cause is a short circuit between the sensor signal wire and chassis ground, which pulls the signal to zero regardless of the actual sensor resistance. A failed sensor whose thermistor has collapsed internally to near-zero resistance produces the same effect. In both cases the PCM cannot derive a valid fuel temperature and substitutes a default value, illuminating the MIL. On diesel engines that depend on accurate fuel temperature for injection quantity calculations, P0187 can produce noticeably rich or lean fuelling, hard cold starts, and elevated smoke at startup. The code is specific to sensor B; the equivalent code for sensor A is P0182.

Common causes

Most-frequently reported root causes when P0187 is logged.

  • 1
    Short circuit to ground on the sensor B signal wire pulling voltage to near zero
  • 2
    Fuel temperature sensor B internal failure with thermistor resistance collapsed to near zero ohms
  • 3
    Corroded or water-contaminated connector creating a conductive path between signal and ground
  • 4
    Chafed wiring harness with the signal wire contacting the vehicle chassis or engine block
  • 5
    Low battery or charging system voltage affecting the PCM reference supply and signal floor
  • 6
    Faulty PCM input circuit reading the signal incorrectly at low voltages (rare)
  • 7
    Water intrusion into the sensor body causing internal short between element and housing

Symptoms drivers notice

Malfunction indicator lamp (MIL) illuminated
Poor fuel economy as the PCM compensates for a falsely indicated extreme fuel temperature
Hard starting, especially in cold weather, due to incorrect cold-start enrichment calculations
Reduced engine performance, particularly during warm-up phase
On diesel engines: possible excessive smoke at startup due to miscalculated injection quantity

How to diagnose P0187

A typical diagnostic flow when this code is present.

  1. 1
    Connect a scan tool and record all codes; note the fuel temperature B live value — a reading stuck at maximum temperature or showing an implausibly high value confirms the low-voltage/short-to-ground condition
  2. 2
    With the ignition on and sensor B connected, back-probe the signal wire and measure voltage to ground; a reading at or near 0 V confirms the signal is shorted to ground
  3. 3
    Disconnect sensor B and re-measure voltage on the signal wire at the harness side; if voltage rises to approximately 5 V with the sensor disconnected, the sensor itself is shorted internally and should be replaced
  4. 4
    If voltage remains near 0 V with the sensor disconnected, the short is in the wiring harness; trace the signal wire for chafe points, pinch points, or contact with grounded metal and repair
  5. 5
    Inspect the sensor connector thoroughly for moisture, corrosion, or contamination bridging the signal and ground terminals; clean and dry the connector before retesting
  6. 6
    Measure sensor B resistance directly across its terminals with it disconnected; near-zero resistance confirms an internally failed sensor
  7. 7
    After repair or sensor replacement, clear codes and confirm with a test drive that the fuel temperature B reading tracks realistically with coolant temperature from cold start through operating temperature

Related powertrain codes

Frequently asked questions

What does "low input" mean for a fuel temperature sensor?

The sensor produces a voltage signal that rises with colder fuel (high resistance = higher voltage) and falls with warmer fuel (lower resistance = lower voltage). "Low input" means the signal voltage is below the minimum plausible value, indicating a short to ground or a completely failed sensor, rather than a value that is merely out of the expected temperature range.

How do I tell if P0187 is caused by a bad sensor or bad wiring?

Disconnect sensor B and measure the signal wire voltage at the harness connector with the ignition on. If the voltage rises to approximately 5 V with the sensor unplugged, the sensor is internally shorted and should be replaced. If the voltage stays near 0 V, the wire itself is shorted to ground somewhere between the connector and the PCM.

Is P0187 related to P0185 and P0186?

All three codes relate to fuel temperature sensor B. P0185 is a general circuit malfunction (open or hard fault), P0186 is a range/performance issue (signal present but implausible), and P0187 specifically identifies a low-voltage condition indicating a short to ground or a collapsed sensor element. P0187 is the B-circuit counterpart to P0182 on sensor A.

Can low battery voltage cause P0187?

Marginally. Severely low battery or charging voltage can reduce the PCM reference supply and shift signal thresholds, but typically not enough to trigger P0187 on its own. If P0187 appears alongside codes for low supply voltage or charging system faults, address those first and confirm whether the fuel sensor code persists before testing the sensor circuit in isolation.

Disabling P0187 in software

RaceTune can permanently disable P0187 — 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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