OBD-II Diagnostic Trouble Code
P0505 — Idle Control System
P0505 indicates an idle control system malfunction: the ECM could not hold the target idle speed. On gasoline trucks it relates to the idle air control valve or electronic throttle; on diesels the idle is fuel-controlled and the code meaning varies by OEM.
Severity 2/5
Truck-relevant
Service soon
Severity
Service soon
Idle Control System
Code
P0505
Category
Powertrain
Subsystem
Fuel
Truck-relevant
Yes
What this means for truckers
Service this week on a Cummins X15, Detroit DD15, PACCAR MX-13. Not an immediate stop, but ignoring it usually leads to a more expensive fix downstream.
Before you replace parts — try this first
P0505 on the Fuel system is most often dirty throttle body or idle air control passages (gas engines) — 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
Idle may be unstable, surging, too high or too low, and the engine may stall when coming to a stop or when loads like A/C or PTO are switched on. Hard start is possible.
Common causes
- CommonDirty throttle body or idle air control passages (gas engines) ($100–$250)
- CommonVacuum leak (gas engines) ($100–$400)
- OccasionalFaulty idle air control valve or electronic throttle body ($200–$800)
- OccasionalWiring or connector fault to the idle actuator ($100–$300)
How to fix
Clean the throttle body and idle air passages and perform an idle relearn
When to use: First step on gas engines
Smoke test the intake for vacuum leaks
When to use: When idle is high or fuel trims are lean
Command the idle actuator with a scan tool and check response
When to use: To verify actuator function
Replace the IAC valve or throttle body
When to use: When it does not respond after cleaning and wiring checks
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
- → P0500 — Vehicle Speed Sensor Malfunction
- → P0501 — Vehicle Speed Sensor Range/Performance
- → P0502 — Vehicle Speed Sensor Low Input
- → P0503 — Vehicle Speed Sensor Intermittent/Erratic/High
- → P0506 — Idle Control System RPM Lower Than Expected
Frequently Asked Questions
Can I drive with code P0505?
P0505 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 P0505 on a commercial truck?
Total repair cost ranges $100–$800 depending on the root cause. The cheapest fix is usually the first one listed below — Dirty throttle body or idle air control passages (gas engines) — 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 P0505 fail a DOT inspection?
P0505 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 P0505 and related codes?
P0505 is closely related to: P0500 (Vehicle Speed Sensor Malfunction), P0501 (Vehicle Speed Sensor Range/Performance), P0502 (Vehicle Speed Sensor Low Input), P0503 (Vehicle Speed Sensor Intermittent/Erratic/High). 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 P0505 is set without the companions, the issue is more isolated.
How do I prevent P0505 from coming back after a repair?
For P0505 (Idle Control System), 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.