Air intakeGeneric OBD-II codeUrgency: Garage soon
Code P0505: idle speed control: system fault
P0505 means: idle control system: malfunction.[1][2][3] The control unit fails to regulate idle: the actual engine speed deviates from the target it calculates itself. The code does not distinguish whether idle is too high or too low (P0506 and P0507 are for that) and does not point to the faulty part. Samarins gives a typical idle of 650 to 750 rpm on most cars as a guide.[1][2]
What code P0505 means
The control unit fails to regulate idle: the actual engine speed deviates from the target it calculates itself. The code does not distinguish whether idle is too high or too low (P0506 and P0507 are for that) and does not point to the faulty part. Samarins gives a typical idle of 650 to 750 rpm on most cars as a guide.[1][2]
Using a Toyota as an example, AutoDTCs describes that the code is stored when the speed moves more than 100 rpm below the target or more than 150 rpm above it (200 with air conditioning) and, at the same time, the idle airflow the control unit has learned is at one of its limits for 5 seconds. Other manufacturers use other thresholds. That source adds that the warning light comes on after two failed driving cycles, and that many other codes (for example P0335, P0120, P0171, P0172 or P0300) disable this monitor, so they should be resolved first.[1]
What controls idle depends on the engine. On engines with electronic throttle, the throttle body is the only idle control. On older or differently designed engines there is a separate idle air control (IAC) valve; AutoDTCs gives some older Nissans as an example, where P0505 more likely points to a dirty or faulty IAC valve.[1][2][3]
Original definition (SAE J2012): Idle Control System Malfunction[1][2][3]
Causes of P0505
From most to least likely, according to the sources.
- 1Dirt and carbon on the throttle body, which restrict the fine air flow needed for idle. AutoDTCs identifies it as the most common cause, and Samarins also lists it.[1][2]
- 2Unmetered air entering the intake: cracked or loose hoses, loose clamps, or a gasket on the intake manifold or throttle body.[1][2]
- 3Faulty PCV valve or crankcase ventilation hoses that leak, have collapsed or are blocked.[1][2]
- 4IAC valve or its motor dirty, sticking or stuck, or idle air passages blocked, on engines that have one. Samarins comments that these valves fail very often.[2][3]
- 5Faulty electronic throttle (motor or position sensor), which skews the control unit's calculations.[1][2]
- 6Damaged wiring or connector on the throttle body or the IAC valve, or no power supply to the idle valve.[2][3]
- 7Idle not relearned after disconnecting or replacing the battery. AutoDTCs warns that after a control unit reset, P0505 may appear while the values are relearned.[1][2]
- 8Accelerator pedal not returning fully, for example because a floor mat is rubbing it: AutoDTCs includes this Toyota factory tip for this code.[1]
- 9Low coolant level, according to Samarins.[2]
Symptoms
- Irregular idle, hunting or lurching instead of settling.[1][2][3]Vibrations: faults that cause it
- The engine stalls at idle or in slow traffic, especially when warm, or stops shortly after starting without touching the accelerator.[1][2][3]Engine stalls: faults that cause it
- Engine fault warning light on after two failed driving cycles.[1][3]Engine warning light: faults that cause it
- Idle too high that does not drop after the engine has warmed up.[1][2]
- Difficulty starting and hesitation on acceleration.[2]Hard to start: faults that cause it
How to diagnose P0505
In order: from the most likely and cheapest to check to the most expensive.
- 1Read all the codes first. Samarins recommends checking any other codes first, and AutoDTCs reminds that many of them disable the idle monitor.[1][2]
- 2Look for manufacturer technical bulletins for your model: Samarins cites several for Honda, Ford and Toyota, some related to the IAC valve or the throttle body.[2]
- 3With the diagnostic equipment, look at the actual idle speed and the target live. If the code appeared just after changing or disconnecting the battery, try the idle relearn procedure, which varies by car. AutoDTCs adds that if the code appears just after a control unit reset, a hot driving cycle should be completed before continuing the diagnosis.[1][2]
- 4Check that the floor mat does not touch the pedal and that the pedal returns fully when released by hand.[1]
- 5Check the intake and the PCV: look for air leaks with a smoke machine, or with propane around the joints (if the speed changes, there is a leak there), and check the PCV hoses for cracks, kinks or collapse.[1][2]
- 6Inspect the connector and harness of the IAC valve or the throttle body, and the coolant level.[2]
- 7Clean the throttle body or the IAC valve if they are dirty, with a specific cleaner and a soft brush, with no abrasives. On engines with electronic throttle, the throttle body initialisation must then be carried out with the diagnostic equipment: AutoDTCs warns that if this is skipped, idle will be erratic and P0505 will return.[1][2]
- 8Replace the IAC valve or the throttle body only if the problem persists after cleaning, or if a manufacturer bulletin recommends it, and relearn the idle again.[2]
Can I keep driving with P0505?
Garage soon
AutoDTCs rates it as medium severity: it is not immediately dangerous, but unstable idle can make the car stall in traffic, which is why it advises repairing it within a week. CarWiki, which reduces it to a fault in the idle valve or its supply, does not rate the severity, but lists the engine stalling at idle or shortly after starting among the symptoms.[1][3]
Documented faults related to P0505
Known faults of specific engines and cars, with their repair cost, the affected years and their sources.
Carbon build-up in the intake: the most serious faults by engine and car
- Medium severityFrequency: Very frequent2.0 TSI / TFSICarbon build-up on the intake valvesBecause the fuel is injected directly into the cylinder, no petrol washes over the intake valves, so oil residue and soot build up and, over the years, restrict airflow and affect engine response.
- Medium severityFrequency: Common1.6 HDiLeaks at the injector washers and carbon build-up in the cylinder headIt is common for the copper washers of the 1.6 HDi injectors to stop sealing: the engine blows gases, carbon forms around the injector and soot gets into the oil. It is cheap to repair, but if left it contaminates the oil and is one of the causes of turbo failure.Repair (garage): €110–€310
- Medium severityFrequency: Common1.6 GDI / T-GDICarbon build-up on the intake valvesBeing pure direct injection, with no port injectors to wash the valves, the 1.6 GDI and T-GDI build up intake carbon with mileage: unstable idle, jerking or loss of power. A design trait of all direct injection, not a manufacturing fault.
- Medium severityFrequency: CommonMZR-CDCarbon on the injectors, EGR valve and intakeUrban use and short journeys encourage carbon to build up on the injectors, EGR valve and intake of the MZR-CD, with loss of power and jerking; documented on the Mazda6 and Mazda3, on both the 2.0 and the 2.2.
Engine sensors: the most serious faults by engine and car
- High severityFrequency: Occasional2.0 DTI and 2.2 DTIVP44 injection pump and air mass meter failureGerman Wikipedia documents recurring failures of the Bosch VP44 injection pump and the air mass meter in the early years of this engine. An owner testimony for this engine describes the pump being replaced at 192,500 km, at a cost the owner calls «prohibitive».
- Medium severityFrequency: Common2.0 TDIClogged particulate filter or faulty pressure sensorThe 2.0 TDI's particulate filter clogs if the car mostly makes short journeys, because it never gets to regenerate; sometimes the fault is just the differential pressure sensor. Cleaning it costs around €200 and replacing it, €800 to €3,000.Repair (garage): €800–€3,000
- Medium severityFrequency: Common1.4 and 1.6 8vCrankshaft position sensor failure (TDC sensor)The crankshaft position sensor (capteur PMH, or TDC sensor) appears repeatedly in the reliability sources for the Clio II, the Kangoo I and the Dacia Logan I with this engine, with hard cold starting or stalling as the main symptoms. It is a cheap part, but the failure can leave you stranded.
- Medium severityFrequency: Common1.4 and 1.6 16vCrankshaft position sensor failureThe crankshaft position sensor (capteur PMH) appears repeatedly in the reliability sources for almost every generation with this engine, with hard cold starting, stalling or the car cutting out unexpectedly. It is a cheap part, but the failure can leave you stranded.
P0505 by make
Documented faults that can trigger P0505 in each make's engines and cars.
Frequently asked questions
Can I drive with code P0505?
You can, but there is a risk of stalling in slow traffic or when stopping. AutoDTCs advises repairing it within a week. If the engine stalls often or becomes dangerous in a traffic situation, take it to a workshop sooner.[1]
What should be done after cleaning the throttle body?
On engines with electronic throttle, the throttle must be initialised or relearned with diagnostic equipment: the ECM has to learn the closed throttle position again. AutoDTCs warns that if this is skipped, idle will be erratic and the code will return. On other cars, the idle relearn procedure varies by manufacturer.[1][2]
Why did P0505 appear just after changing the battery?
Disconnecting the battery erases the idle values the control unit had learned. Samarins recommends trying an idle relearn if the code appears shortly after changing or disconnecting the battery, and AutoDTCs warns that it may appear while the values are relearned.[1][2]
What is the difference between P0505, P0506 and P0507?
P0505 is the general idle control fault, which according to Samarins can mean idle is either too high or too low. P0506 (speed lower than expected) and P0507 (higher) are more specific: they say which way the idle deviates. AutoDTCs lists P0505 and P0507 as codes related to P0506.[2]
Will I pass the MOT with code P0505?
If the code has switched on the engine warning light, no: with that light on, the inspection records a serious defect, and on Euro 5 and Euro 6 cars the ITV (Spain's roadworthiness inspection, similar to the MOT) also reads the OBD diagnostic system. Repair the fault before you go; clearing the code just before the inspection does not help either, because it counts as a serious defect.[4]
Not sure what is wrong with your car? Describe what you notice in the symptom diagnosis or see all fault codes.
Sources
Every fact on this page links to its source. If you find an error, let us know.
- P0505: Idle Air Control SystemAutoDTCs · Technical documentation · consulted 31 October 2026
- P0505 Idle Control System MalfunctionSamarins.com · Technical documentation · consulted 31 October 2026
- OBD Fehlercode P0505CarWiki.de · Technical documentation · consulted 31 October 2026 · in German
- Desmontando mitos: ¿puede pasar mi coche la ITV con el testigo de «avería motor» encendido?Diariomotor · Motoring press · published 8 March 2021 · consulted 2 October 2026 · in Spanish