P0185
Fuel Temperature Sensor B Circuit MalfunctionP0185 is a generic OBD-II powertrain diagnostic trouble code: Fuel Temperature Sensor B Circuit Malfunction. 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.
What P0185 means
P0185 is the sensor B equivalent of P0180 and indicates that the PCM has detected a general circuit malfunction on the second fuel temperature sensor. Some vehicles, particularly larger diesel engines, turbodiesel applications, and high-performance direct-injection engines, use two fuel temperature sensors: sensor A is typically located upstream of the fuel filter or on the fuel rail, while sensor B may be positioned at the fuel return line, the fuel cooler outlet, or a secondary fuel metering point. The PCM compares readings from both sensors as a plausibility cross-check; a persistent out-of-range or missing signal from sensor B triggers P0185. As with P0180, the PCM will substitute a programmed default temperature value and continue to operate the engine, but fuel metering precision is compromised. Hard starting in cold conditions and degraded fuel economy are the most common practical consequences. On vehicles that do not use a second dedicated fuel temperature sensor, P0185 may indicate a fault in a flex-fuel sensor or a sensor that serves a dual role for both fuel temperature and ethanol content detection.
Common causes
Most-frequently reported root causes when P0185 is logged.
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1
Defective fuel temperature sensor B (thermistor failure or calibration drift)
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2
Corroded, broken, or water-damaged wiring in the sensor B circuit
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3
Loose, corroded, or damaged connector at sensor B or the PCM harness
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4
Short circuit to ground or open circuit in the sensor B signal wire
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5
Faulty PCM or damaged PCM input circuit for sensor B (rare)
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6
Incorrect replacement sensor installed with wrong resistance characteristics
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7
On flex-fuel vehicles: failed or contaminated flex-fuel/ethanol content sensor
Symptoms drivers notice
How to diagnose P0185
A typical diagnostic flow when this code is present.
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1
Connect an OBD-II scan tool, record all codes, and check freeze-frame data to identify conditions at fault onset
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2
Locate sensor B using the OEM wiring diagram — its physical position differs by make and model; common locations include the fuel return line, fuel cooler, or secondary fuel rail
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3
Inspect the sensor B wiring harness and connector for corrosion, water damage, chafing, or loose terminals; repair any physical defects
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4
With the ignition on, back-probe the sensor B connector to confirm the PCM is delivering a 5 V reference and a clean ground return
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5
Disconnect sensor B and measure resistance across its terminals; compare against the OEM temperature-resistance specification at current ambient temperature
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6
Monitor live sensor B data on the scan tool at cold start and compare its reading against sensor A and ambient temperature; implausible or frozen values confirm a fault
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7
If wiring and connectors are intact and sensor resistance is out of specification, replace sensor B and verify the fault does not return
Related powertrain codes
- P0100 — Mass or Volume Air Flow A Circuit Malfunction
- P0101 — Mass or Volume Air Flow A Circuit Range/Performance Problem
- P0102 — Mass or Volume Air Flow A Circuit Low Input
- P0103 — Mass or Volume Air Flow A Circuit High Input
- P0104 — Mass or Volume Air Flow A Circuit Intermittent
- P0105 — Manifold Absolute Pressure/Barometric Pressure Circuit Malfunction
Frequently asked questions
What is the difference between fuel temperature sensor A and sensor B?
Sensor A is typically the primary fuel temperature measurement point, most often located on the fuel rail or at the fuel filter inlet. Sensor B is a secondary measurement point, commonly placed at the fuel return line or a fuel cooler outlet. The PCM uses both readings for precise fuel metering and as a plausibility cross-check against each other.
Does every vehicle have a sensor B?
No. Many petrol vehicles use only one fuel temperature sensor (sensor A). Sensor B is most common on multi-circuit diesel engines, flex-fuel vehicles (where it may serve as the ethanol content sensor), and some direct-injection petrol engines with fuel return circuits. If your vehicle only has one sensor, P0185 is unlikely unless the PCM firmware assigns a dual role to a single sensor.
Can P0185 affect emissions?
Yes. If the PCM is forced to use a default fuel temperature and the actual temperature differs significantly, injection quantity errors can push the air-fuel mixture off its target, increasing hydrocarbons and particulate output. On vehicles with strict OBD monitoring, this may cause an emissions test failure.
Is P0185 serious enough to prevent driving?
In most cases the vehicle remains drivable, as the PCM substitutes a default value and maintains basic engine function. However, driving for extended periods with degraded fuel metering precision increases fuel consumption and can stress the catalytic converter or diesel particulate filter, so the fault should be diagnosed and repaired promptly.
Disabling P0185 in software
RaceTune can permanently disable P0185 — 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.
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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