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P0186 – Fuel Temperature Sensor B Circuit Range/Performance (OBD2)

P0186 means "Fuel Temperature Sensor B Circuit Range/Performance". The fuel composition / fuel temperature sensor signal is electrically present but implausible: it does not match what the control unit expects for the current operating conditions or from other sensors. This is a performance fault, not a broken wire.

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P0186 quick summary

Fault codeP0186
MeaningFuel Temperature Sensor B Circuit Range/Performance
SystemPowertrain (Engine & Transmission)
VehiclesGeneric OBD2
SeverityLow
Can I drive?Yes, in most cases. Power or starting may be slightly affected. If P0168 (fuel temperature too high) appears, avoid running the tank very low and have the fuel cooling checked.
Repair cost€20 – €400

How fuel composition and fuel temperature sensors work

These sensors tell the engine control unit the fuel's temperature and, on flex-fuel vehicles, its ethanol content, so it can correct injection quantity and timing.

Fuel temperature sensors are mostly used on diesels and some direct-injection engines: warm fuel is less dense, so the control unit adjusts the injected quantity. Flex-fuel vehicles use a fuel composition sensor that measures the ethanol percentage and fuel temperature in the supply line.

Codes in this group cover the sensors' electrical circuits and performance, an engine fuel temperature that is too high (P0168) and incorrect fuel composition (P0169). Very hot fuel can be caused by a faulty fuel cooler or by running the tank very low in hot weather.

A faulty fuel temperature signal usually causes mild symptoms – slightly reduced power or harder starting – while a wrong ethanol reading on a flex-fuel car can cause hard starting and poor running.

On VAG vehicles (VW, Audi, Škoda, Seat, Cupra), VCDS and older VAG testers can show this code as the five-digit VAG code 16570 (P0186). Both numbers mean the same fault.

Symptoms of P0186

  • Check Engine light on
  • Slightly reduced power
  • Harder hot or cold starting
  • Higher fuel consumption
  • Often no noticeable symptoms

Most common causes of P0186 (most likely first)

Mechanic's Pro Tip

On diesels with P0168, check injector return flow: worn injectors that leak a lot of fuel back to the tank heat up the fuel – replacing the temperature sensor will not cure that.

  1. 1.Fuel composition / fuel temperature sensor worn, contaminated or out of calibration

    Most likely

    The signal drifts slowly, so it is still within the electrical limits but wrong.

  2. 2.Mechanical problem in the system the fuel composition / fuel temperature sensor measures or controls

    For example a leak, a sticking part or a restriction that really changes the measured value.

  3. 3.High resistance in a connector or ground

    Corrosion causes a voltage drop that shifts the reading.

  4. 4.Failed fuel temperature or composition sensor

  5. 5.Fuel genuinely too hot (P0168)

    Faulty fuel cooler, very low fuel level in hot weather or an excessive return flow from leaking injectors.

  6. 6.5 V reference or ground problem

Step-by-step diagnostic guide

  1. 1

    Read codes and live data

    Read all codes and compare the fuel temperature in live data with coolant and intake air temperature on a cold engine.

  2. 2

    Compare live data with expected values

    Read the fuel composition / fuel temperature sensor values in live data and compare them with related readings (engine speed, load, temperature or pressure) and with the specification. A value that is electrically fine but wrong for the conditions confirms a performance fault – then check the mechanical side before replacing the part.

  3. 3

    Measure the sensor

    Measure resistance (thermistor) at a known temperature or check the frequency signal of a composition sensor.

    Multimeter: Thermistor sensors: resistance falls as temperature rises; compare with the specification table.

  4. 4

    Check wiring

    Check the reference, ground and signal wires for opens and shorts.

  5. 5

    Check fuel quality and cooling

    Check for water in the fuel, the correct fuel type and – for P0168 – the fuel cooler and return flow.

  6. 6

    Replace and verify

    Replace the sensor or fix the cause and clear codes.

Frequently asked questions

Can I keep driving with P0186?

Yes, in most cases. Power or starting may be slightly affected. If P0168 (fuel temperature too high) appears, avoid running the tank very low and have the fuel cooling checked.

How much does it cost to fix P0186?

Typical repair costs are 20 € – 400 €, depending on the cause and the vehicle. The most common fixes are: fuel composition / fuel temperature sensor worn, contaminated or out of calibration; mechanical problem in the system the fuel composition / fuel temperature sensor measures or controls; high resistance in a connector or ground. Always confirm the cause with the tests above before buying parts.

What does P0169 incorrect fuel composition mean?

The control unit measured a fuel composition it does not expect – usually on flex-fuel vehicles with a wrong ethanol content, or with water or contamination in the fuel.

Why does fuel temperature matter?

Warm fuel is less dense, so the same injection time delivers less fuel. The control unit corrects for this using the fuel temperature sensor.

What is the difference between P0186 and similar codes?

P0186 belongs to a group of codes for the same component that differ in the type or location of the fault: P0183 – Fuel Temperature Sensor A Circuit High Input; P0184 – Fuel Temperature Sensor A Circuit Intermittent; P0185 – Fuel Temperature Sensor B Circuit Malfunction; P0187 – Fuel Temperature Sensor B Circuit Low Input; P0188 – Fuel Temperature Sensor B Circuit High Input; P0189 – Fuel Temperature Sensor B Circuit Intermittent. The tests on this page apply to the whole group; the type of fault tells you where to start.

Related fault codes

This guide is for information only. Working on vehicles can be dangerous — if you are unsure, have the car checked by a qualified mechanic. Values can differ between model years; always check the manufacturer's repair information.