OBD-II trouble code
P0187: Fuel Temperature Sensor "B" Circuit Low
Signal voltage has fallen near zero, which decodes as impossibly hot fuel. On vehicles where this sensor lives in the tank module, the fuel gauge is the first thing to look at — one bad connector explains both faults.
Quick facts
- System
- Powertrain
- Category
- Fuel & Air
- Severity
- Low severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $0 – $800
- DIY difficulty
- Intermediate DIY
What does P0187 mean?
A fuel temperature sensor is a thermistor, and on the negative-coefficient type used almost everywhere, heat drives resistance down. Less resistance pulls the signal line closer to ground, so low voltage decodes to high temperature. P0187 therefore appears on a scan tool as fuel that is far hotter than it could possibly be — a hundred and forty degrees Celsius on a vehicle parked all night, or a reading pinned at the top of the table from the moment the key turns. Nothing is hot. Something has connected the signal wire to ground, or the sensor has failed short internally.
What makes the "B" channel different from its "A" counterpart is not the physics but the company it keeps. On a large share of applications, the second fuel temperature sensor is the tank-side one, built into the fuel pump and sender module that sits on top of the tank. That puts its signal wire into the same connector, and often the same short loom, as the fuel pump — a high-current load that on modern vehicles is driven by a pulse-width-modulated control module rather than a simple relay. It also puts it alongside the fuel level sender. Three circuits, one connector, one ground path, sitting in a place nobody inspects.
That physical arrangement produces a diagnostic shortcut that is worth taking before any measurement. Look at the fuel gauge, and ask the owner about it. If the gauge has been erratic, sticking, or reading wrong at the same time as this code appeared, you are almost certainly not looking at two faults. You are looking at one corroded module connector or one degraded ground affecting everything that passes through it. The same is true in the other direction: a low-voltage fuel temperature fault together with intermittent pump behaviour or a pump-circuit code points at the shared harness rather than at a thermistor that costs a fraction as much to replace and will not fix anything.
The ground path deserves specific attention here. The fuel pump's return current is one of the larger loads on the vehicle and its ground is frequently a body stud under the rear floor, in the worst possible place for road salt and standing water. When that ground develops resistance, pump current finds other routes back, and the sensor ground sharing the same connector is one of them. The symptom that gives it away is that the false temperature reading changes when the pump load changes — it shifts as the pump ramps up under acceleration, or when the tank is nearly empty and the pump is working hardest. A sensor shorted internally does not care what the pump is doing. A shared-ground fault does.
The test that separates sensor from harness is quick and should be done before ordering anything. Unplug the sensor connector and watch the reported fuel temperature. With the circuit open, a healthy harness sends the reading to the cold extreme of the table. If it swings cold, the short is inside the sensor. If it stays pinned hot with nothing connected, the sensor is not the problem and the fault lies between that connector and the module — and on a tank-mounted installation, that saves you from dropping a tank for no reason.
Common causes
- Signal wire shorted to ground or to the sensor ground wire inside the loom
- Corroded or contaminated fuel pump module connector bridging adjacent terminals
- Degraded rear body ground shared with the fuel pump, shifting the sensor reference
- Sensor failed short internally
- Chafed harness where it crosses the underbody, a tank strap or a frame rail
- Water sitting in a low-mounted connector after a failed seal
- Terminal pushed back in the connector and contacting the housing or a neighbouring pin
- Damage from previous fuel pump, sender or tank work leaving a wire pinched
Symptoms
- Check engine light with fuel temperature reported at the top of the scale on a cold engine
- Fuel gauge behaving erratically, sticking, or reading incorrectly at the same time
- Reported fuel temperature that shifts as the fuel pump loads up under acceleration
- Slightly reduced power or economy on diesel applications that derate on hot fuel
- Intermittent fuel pump behaviour alongside the temperature fault
- Vehicle otherwise driving normally
- Emissions inspection rejected on the stored code
Diagnostic steps
- 1.Read live fuel temperature on an engine that has been parked overnight. A value at the top of the scale on a cold vehicle confirms the fault is present now rather than historic.
- 2.Ask about the fuel gauge before touching anything. A gauge fault appearing at the same time as this code points at one shared connector or ground rather than at two separate failures, and it changes where you start.
- 3.Check for fuel pump or fuel level sender codes stored alongside it. Codes clustering around the tank module are strong evidence for the shared harness.
- 4.Locate the sensor for your application before assuming it is in the tank. Confirm rather than infer, because the whole approach differs between a tank module and a line-mounted sensor.
- 5.Unplug the sensor connector and watch the reading. It should swing to the cold extreme. If it does, the short is inside the sensor.
- 6.If the reading stays pinned hot with the connector unplugged, the short is in the harness between there and the module. Do not drop a tank to replace a sensor at this point.
- 7.With the connector unplugged, measure resistance from the signal wire to chassis ground. A low reading confirms the short and identifies the section to isolate.
- 8.Watch whether the false reading moves as pump load changes — during a hard acceleration, or with a nearly empty tank. Movement points at a shared ground rather than a shorted sensor.
- 9.Inspect and clean the rear body ground shared with the fuel pump, then retest before condemning any component.
Repair cost
$0 – $800
Diagnosis is $80 to $160. A harness, terminal or ground repair — the most likely outcome when this code involves a tank module — runs $90 to $280. A line- or rail-mounted sensor is $70 to $250 fitted. A sensor built into the pump and sender module is far more, at $250 to $800 with the tank dropped, and on some vehicles the thermistor is not sold separately from the module.
Estimate your repair
Run the numbers for your vehicle
Open the Repair Cost Estimator with wiring harness / circuit repair preselected. Adjust labor rate and vehicle category to fit your situation.
DIY vs shop
This is an intermediate DIY job. It usually involves diagnostic steps, specialty parts, and some careful work in tight spaces. If you have the tools and a service manual or trustworthy video for your specific vehicle, it is achievable in a weekend. Otherwise, a competent independent shop will be faster.