When a fuel temperature sensor reports fuel hotter than boiling water, the engine computer knows something is wrong with the wiring or the sensor. A low voltage from sensor “B” means just that. A P0187 code means the fuel temperature sensor B signal voltage is too low.
This guide explains what pulls the voltage down, how to test the circuit and which repairs fix the code.
P0187 Code Breakdown
Every OBD-II code follows the same five-character pattern set by the SAE J2012 standard. Each character narrows down which system has the fault. This table shows what each part of the code tells you.
| Character | Meaning | What It Tells You |
|---|---|---|
| P | Powertrain | The fault is in the engine or transmission systems. |
| 0 | Generic (SAE) code | The definition is the same on every OBD-II vehicle. |
| 1 | Fuel and air metering | The fault involves the sensors that measure air and fuel. |
| 87 | Specific fault | Fuel Temperature Sensor B Circuit Low Input |
What the P0187 Code Means
The SAE J2012 standard defines the P0187 code as Fuel Temperature Sensor B Circuit Low Input. Sensor B is the second fuel temperature sensor on vehicles that use two.
It works like other engine thermistors. Its resistance falls as fuel heats up, which pulls the signal voltage down. The powertrain control module (PCM) reads a low voltage as hot fuel. When the voltage drops close to 0 volts, the scan tool shows an impossibly high temperature, often 250°F or more.
A signal that low usually means the signal wire is shorted to ground or the sensor has shorted inside. The PCM then uses a default value or relies on sensor A.
Signs of a Low Sensor B Signal
Many drivers see only the warning light. Common signs of a P0187 code include:
- Check engine light on
- Fuel temperature B reading extremely hot on the scan tool
- Reduced power on some diesels
- Hard starting or long cranking
- Slight rough running
What Causes the P0187 Code?
Low input faults almost always come from a short. These are the usual causes:
- Signal wire shorted to ground. Chafed insulation touching metal pulls the voltage down.
- Internally shorted sensor B. The thermistor fails with near-zero resistance.
- Fuel or moisture in the connector. Contamination bridges the pins.
- Damaged wiring near hot parts. Heat and vibration wear through insulation.
- PCM fault. Rare.
How to Test the Circuit
You need a scan tool, a digital multimeter and the wiring diagram.
Confirm the Hot Reading
With the key on, compare fuel temperature B with sensor A and the coolant temperature. A sensor B reading far hotter than the others, especially on a cold engine, confirms the fault.
Do the Unplug Test
Unplug sensor B with the key on and watch the scan tool. The reading should drop to about -40°F. If it does, the wiring is fine and the sensor has shorted inside. If it stays hot, the signal wire is shorted to ground somewhere between the connector and the PCM.
Check for a Short
Disconnect the battery and unplug the sensor. Check for continuity between the signal wire and ground. Any continuity means a short. Inspect the wiring along its route, especially near clamps, brackets and the exhaust.
Repairs That Clear the P0187 Code
Repair what your tests found. Then clear the code and confirm a normal reading in live data.
- Replace sensor B if the reading drops to -40°F when unplugged.
- Repair a shorted signal wire and protect it from rubbing.
- Clean and dry the connector if fuel or water got inside.
- Fix any fuel leak that soaked the connector.
Relieve fuel pressure before you remove the sensor. On diesels and direct injection engines, follow the service manual, since pressure can be very high.
When a short to ground is the cause, the damage is often where the wiring rubs against a bracket or passes near the exhaust. Look for shiny spots where the insulation has worn through. Repair the wire and add protective loom so it cannot rub again.
Repair Cost Breakdown
Most P0187 code fixes are moderate. This table shows typical ranges.
| Repair | DIY Cost | Typical Shop Cost |
|---|---|---|
| Wiring repair | $10 to $30 | $100 to $300 |
| Connector cleaning or pigtail | $10 to $40 | $75 to $200 |
| Fuel temperature sensor B | $20 to $120 | $100 to $300 |
| Sensor built into a pump or filter housing | $80 to $400 | $250 to $800 |
Is It Safe to Drive with a P0187 Code?
You can usually drive carefully. The PCM uses a default value or sensor A.
A false hot reading may make some diesels cut power to protect the fuel system. Get the circuit fixed soon to restore normal performance.
Related Codes
These codes often show up with this one or point to the same system:
| Code | Meaning |
|---|---|
| P0184 | Fuel Temperature Sensor A Circuit Intermittent |
| P0185 | Fuel Temperature Sensor B Circuit Malfunction |
| P0186 | Fuel Temperature Sensor B Circuit Range/Performance |
| P0188 | Fuel Temperature Sensor B Circuit High Input |
| P0189 | Fuel Temperature Sensor B Circuit Intermittent |
Fixing a Low Sensor B Signal
A P0187 code means fuel temperature sensor B voltage is too low, so the PCM sees impossibly hot fuel. Shorted sensors and shorted wires cause most cases.
- Compare sensor B with sensor A and coolant temperature.
- Unplug the sensor and watch for -40°F.
- Check the signal wire for a short to ground.
- Replace sensor B if the wiring tests good.
Frequently Asked Questions
Why Does a Short Make the Fuel Read Hot?
The sensor’s resistance drops as fuel heats up, which lowers the signal voltage. A short to ground drops the voltage to nearly zero. The PCM reads that as the hottest possible temperature, even though the fuel is fine.
Can I Swap the Two Sensors to Test?
Only if they are the same part and fit the same way. If they are, swapping them can show whether the fault follows the sensor. Many vehicles use different parts in each position, so check the part numbers first.
Is the P0187 Code Related to P0182?
They are the same type of fault on different sensors. P0182 is low input on sensor A, and the P0187 code is low input on sensor B. If both appear together, look for a shared ground or reference problem.
Will This Code Damage the Engine?
The code itself will not damage the engine. However, some diesels reduce power when they lose a reliable fuel temperature reading. Fixing the circuit restores full power and lets the PCM protect the fuel system properly.
Why Is Fuel Leaking Near the Sensor?
Sensors mounted in fuel lines or housings seal with an O-ring or a crush washer. As the seal ages, it can seep. Fuel then runs into the connector and can short the pins. Replace the seal or sensor, clean the connector and recheck the code.