P0185

What Does Code P0185 Mean?

DTC P0185 signifies a “Fuel Temperature Sensor B Circuit Malfunction.” This code is set by the Powertrain Control Module (PCM) or Engine Control Module (ECM) when it detects an electrical fault within the circuit of Fuel Temperature Sensor B. Fuel Temperature Sensor B is typically a Negative Temperature Coefficient (NTC) thermistor, meaning its electrical resistance decreases as the fuel temperature increases. The PCM supplies a reference voltage (commonly 5V) to the sensor and monitors the voltage drop across it to determine the fuel temperature. When the PCM detects a voltage signal that is outside the expected operating range (either too high, indicating an open circuit or implausibly cold fuel, or too low, indicating a short circuit or implausibly hot fuel), or if the signal is inconsistent with other vehicle operating parameters (e.g., ambient air temperature, engine coolant temperature after a cold soak), it interprets this as a circuit malfunction and sets P0185.

The Fuel Temperature Sensor B provides critical data to the PCM for accurate fuel management strategies. This includes calculating fuel density, which directly impacts fuel mass and, consequently, injector pulse width for precise air-fuel ratio control. It also plays a role in evaporative emissions (EVAP) system operation, especially concerning fuel vapor pressure and purge strategies. A malfunction in this circuit can lead to incorrect fuel mixture calculations, affecting engine performance, fuel economy, and emissions.

Common Symptoms

• Malfunction Indicator Lamp (MIL) illumination: The “Check Engine” light will be illuminated on the dashboard.
• Decreased fuel efficiency: Incorrect fuel density calculations can lead to either an overly rich or lean fuel mixture, consuming more fuel.
• Rough idle or poor engine performance: Erratic fuel delivery due to inaccurate temperature readings can cause misfires, hesitation, or a general lack of power.
• Difficulty starting: Especially in extreme ambient temperatures, the engine may crank longer or struggle to start if fuel temperature compensation is incorrect.
• Failed emissions test: An incorrect fuel mixture or compromised EVAP system operation can lead to excessive emissions.
• Engine may enter “limp mode”: In some vehicle applications, a severe or persistent circuit malfunction may prompt the PCM to enter a protective mode, limiting engine performance to prevent potential damage.

What Causes the Code P0185?

• Faulty Fuel Temperature Sensor B: The sensor itself may have an internal open circuit, short circuit, or degradation of its thermistor element, causing it to send an inaccurate or no signal.
• Wiring harness issues:
  • Open circuit: A break in the signal, reference, or ground wire preventing signal transmission.
  • Short to ground: The signal wire making unintentional contact with chassis ground, leading to a consistently low voltage.
  • Short to voltage: The signal wire making unintentional contact with a power source, leading to a consistently high voltage.
  • High resistance: Corrosion or damage within the wiring causing an excessive voltage drop or signal distortion.
• Corroded or damaged electrical connectors: Poor pin tension, water intrusion, or oxidation at either the sensor connector or the PCM/ECM connector can disrupt the circuit integrity.
• PCM/ECM failure: While less common, an internal fault within the Powertrain Control Module (PCM) or Engine Control Module (ECM) could cause this code by incorrectly interpreting the sensor signal or failing to provide proper reference voltage/ground.

How to Diagnose and Troubleshoot

Diagnosis of P0185 requires a methodical approach, utilizing an OBD-II scanner and a digital multimeter (DMM).

1. Retrieve DTCs and Freeze Frame Data: Connect an OBD-II scanner and retrieve all stored Diagnostic Trouble Codes (DTCs). Note any other related codes that might be present. Review the freeze frame data associated with P0185, as this provides a snapshot of engine conditions (engine speed, load, coolant temp, etc.) at the time the code was set.
2. Monitor Live Data Stream: With the scanner, observe the live data stream for “Fuel Temperature B.” Compare its reading to other temperature sensors (e.g., Intake Air Temperature (IAT), Engine Coolant Temperature (ECT)) after a cold soak (vehicle sitting overnight) for plausibility. An implausible reading (e.g., -40°C, 150°C, or a fixed default value) strongly suggests a circuit issue or a faulty sensor.
3. Visual Inspection:
   • Locate Fuel Temperature Sensor B. On many vehicles, this sensor is integrated into the fuel pump module assembly inside the fuel tank or is located on the fuel feed line.
   • Inspect the sensor and its electrical connector for obvious signs of damage, corrosion, loose connections, or broken lock tabs.
   • Carefully trace the wiring harness back from the sensor towards the PCM/ECM, checking for chafing, cuts, pinches, or signs of rodent damage, especially where the harness passes through bulkheads or near sharp edges.
4. Electrical Circuit Testing (using a DMM):
   • Check Reference Voltage: With the ignition ON (engine OFF) and the sensor connector disconnected, use a DMM to measure voltage between the reference voltage pin (typically 5V, consult wiring diagram for exact pinout) and a known good chassis ground. The reading should be within 4.8V-5.2V. If no voltage or incorrect voltage, suspect wiring from PCM or PCM internal fault.
   • Check Ground Circuit: Verify continuity between the sensor’s ground pin and chassis ground (or the PCM’s sensor ground pin, as per wiring diagram). Resistance should be less than 5 ohms. An open ground circuit will cause the sensor to read high or open.
   • Check Signal Circuit for Open/Short:
     • Disconnect both the sensor connector and the PCM/ECM connector.
     • Measure continuity between the signal wire pin at the sensor connector and its corresponding pin at the PCM/ECM connector. Resistance should be less than 5 ohms. High resistance indicates a wiring issue.
     • Check for a short to ground by measuring resistance between the signal wire and chassis ground. This should be infinite (open circuit) or very high resistance.
     • Check for a short to voltage by measuring resistance between the signal wire and the B+ wire or other power wires. This should also be infinite.
   • Test Sensor Resistance (if accessible and removable): If the sensor can be removed or its terminals safely accessed, measure its internal resistance with the DMM. Compare this reading to manufacturer specifications (often provided as a resistance-vs.-temperature chart). Gently heat or cool the sensor in a controlled environment (e.g., using a heat gun or ice water bath) to observe if its resistance changes appropriately. An open circuit (infinite resistance) or a short circuit (near zero resistance) indicates a faulty sensor.

Recommended Repairs and Solutions

Based on the diagnostic findings, the following repairs are typically recommended:

1. Replace Fuel Temperature Sensor B: If electrical testing confirms the sensor itself is faulty (e.g., internal open, short, or out-of-spec resistance), replacement is necessary. This often involves accessing the fuel pump module inside the fuel tank, which can be a time-consuming procedure requiring specialized tools to remove the tank or access panel. Always use an OEM equivalent or better quality replacement part.
2. Repair or Replace Wiring Harness: If the DMM tests pinpoint an open circuit, short to ground, short to voltage, or excessive resistance in the wiring, the damaged section of the harness must be repaired. Use appropriate automotive-grade wiring, solder connections (preferred for durability), and marine-grade heat shrink tubing to seal repairs from moisture. If the damage is extensive or affects multiple wires, replacement of the entire harness section may be more practical and reliable.
3. Clean or Replace Electrical Connectors: If corrosion or poor pin tension is identified at either the sensor or PCM/ECM connector, first attempt to clean the terminals using specialized electrical contact cleaner and a small wire brush or pick. If the corrosion is severe, or pins are bent/broken, the connector shell and/or individual pins should be replaced. Ensure the connector seals properly to prevent future moisture intrusion.
4. PCM/ECM Replacement/Reprogramming: This should be considered a last resort, only after all other potential causes (sensor, wiring, connectors) have been thoroughly inspected, tested, and ruled out. If a new PCM is installed, it will almost always require programming or “flashing” with the vehicle’s specific software, which typically requires a dealership-level diagnostic tool or specialized aftermarket equipment.

After any repair, clear the DTCs with the OBD-II scanner and perform a comprehensive test drive under varying conditions to ensure the code does not return and that the vehicle’s fuel management system is operating correctly.