OBD-II Diagnostic Trouble Code
P0186 — Fuel Temperature Sensor B Circuit Low
P0186 means the fuel temperature sensor B signal voltage is below the ECM's minimum, which normally reads as an extremely hot fuel temperature. Usually the signal wire is shorted to ground or the sensor is shorted internally.
Severity 2/5
Truck-relevant
Service soon
Severity
Service soon
Fuel Temperature Sensor B Circuit Low
Code
P0186
Category
Powertrain
Subsystem
Fuel
Truck-relevant
Yes
What this means for truckers
P0186 (Fuel Temperature Sensor B Low) — secondary fuel temp sensor shorted to ground. Replace sensor or repair wiring short.
Before you replace parts — try this first
P0186 on the Fuel system is most often signal wire shorted to ground — not a dead part. On commercial diesels especially, work these in order before spending money on parts:
- Confirm the symptoms. Match what your truck is doing to the Symptoms section below. If the code is set but the symptoms are not there, clear the code and re-test after a drive cycle — it may not return.
- Clean and dry the sensor connector. Water, road salt, or oil seeping into a connector is the #1 fake sensor failure. Disconnect, look for green corrosion or moisture, spray with electrical contact cleaner, dry, reseat. Many faults clear here for $0.
- Inspect the wiring harness. Trace the wire from the sensor toward the ECM — look for chafing against the engine block, melted insulation near the turbo or exhaust, or pinch points where a clamp has rubbed through.
- Test the sensor before swapping it. Most Common Causes (below) list the expected voltage or resistance range — a $20 multimeter saves a $300 sensor swap that does not fix the problem.
- Only then, replace the part. Use the OEM or a tier-1 aftermarket. Cheap sensors throw the same code in two weeks.
Symptoms
Check-engine light is on, and the scan tool shows an unrealistic high temperature for sensor B. On diesels the ECM may derate to protect the injection system, so you may notice reduced power.
Common causes
- CommonSignal wire shorted to ground ($100–$350)
- CommonInternally shorted sensor B ($80–$300)
- OccasionalFluid inside the connector ($50–$250)
How to fix
Unplug sensor B and watch the reading. A jump to cold means the sensor is shorted.
When to use: First test
Check the signal wire for a short to ground and inspect the connector.
When to use: When the reading stays hot with the sensor unplugged
Replace sensor B or repair the wiring.
When to use: Based on the results
Affects most
No specific vehicle pattern found in our complaint database. This code can appear on any vehicle that supports OBD-II diagnostics.
Related
- → Browse all DTC trouble codes
- → Decode a VIN to check codes for your truck
- → Owner-operator maintenance guides
- → P0100 — Mass or Volume Air Flow Circuit
- → P0101 — Mass or Volume Air Flow Circuit Range/Performance
- → P0102 — Mass or Volume Air Flow Circuit Low Input
- → P0103 — Mass or Volume Air Flow Circuit High Input
- → P0104 — Mass or Volume Air Flow Circuit Intermittent
Frequently Asked Questions
Can I drive with code P0186?
P0186 is severity 2 — service this week. Not an immediate stop, but ignoring it usually compounds the problem. Most common cause for this code is a worn sensor or minor performance fault that won't strand you today but will get more expensive the longer you wait.
How much does it cost to fix P0186 on a commercial truck?
Total repair cost ranges $50–$350 depending on the root cause. The cheapest fix is usually the first one listed below — Signal wire shorted to ground — which is also the most common. On commercial trucks the cost is usually higher than passenger vehicles due to part availability and shop labor rates ($120–185/hour at most truck shops, vs $90–130 at general auto shops). DIY-capable fixes (sensors, hoses, filters) are at the low end; major component replacements (turbo actuator, high-pressure fuel pump, transmission solenoid pack) are at the high end. Always diagnose with a scan tool BEFORE replacing parts — guessing wrong on a $1,500 turbo actuator when the actual issue was a $30 boost hose is a common expensive mistake.
Will P0186 fail a DOT inspection?
P0186 itself doesn't automatically fail an inspection, but if the truck is in reduced-power mode (limp mode) at the time of inspection, it WILL be flagged. Most inspectors will note the check engine light but pass the truck if it's running and performance is normal. Fix it before a Level 1 inspection regardless.
What's the difference between P0186 and related codes?
P0186 is closely related to: P0100 (Mass or Volume Air Flow Circuit), P0101 (Mass or Volume Air Flow Circuit Range/Performance), P0102 (Mass or Volume Air Flow Circuit Low Input), P0103 (Mass or Volume Air Flow Circuit High Input). These codes share the same subsystem (fuel) and often appear together. If you see this code, scan for the full code list — fixing the root cause usually clears multiple companion codes at once. If only P0186 is set without the companions, the issue is more isolated.
How do I prevent P0186 from coming back after a repair?
For P0186 (Fuel Temperature Sensor B Circuit Low), prevention centers on the fuel system. Replace fuel filters at the OEM interval — don't stretch them. Use only diesel from high-volume fuel stops (avoid bottom-of-tank fuel at low-traffic stations). Add a water separator if you don't have one. After repair, prime the fuel system per the engine manual.