P0505 on 2002-2006 Mercedes-Benz E55 AMG: Idle Control System Causes and Fixes

What's Unique About the 2002-2009 Mercedes-Benz E-Class

The supercharged M113K engine has a complex network of vacuum and breather hoses packed into a tight space at the rear of the engine. These rubber and plastic lines become brittle with age and heat, making vacuum leaks the number one cause for P0505. It's especially common for a hose to be left disconnected or to crack during other services like changing valve cover gaskets. Additionally, this code is a frequent issue for owners who have installed larger, aftermarket throttle bodies (e.g., 82mm), as the factory ECU tune struggles to adapt without specific recalibration using a tool like Mercedes STAR/XENTRY.

Generation note: The specified engine, the supercharged M113K E55 5.4L V8, was available in the E-Class across two generations: the final 2002 model year (W210 chassis) and the 2003-2006 model years (W211 chassis). The prompt's range extends to 2009, but the E55 was replaced by the E63 AMG (with the M156 engine) in 2007. The causes and fixes are broadly similar for both the W210 and W211 E55 models as they share the same fundamental engine and 'drive-by-wire' throttle architecture.

Symptoms You May Notice

• Check Engine Light is on
• Rough, erratic, or 'lumpy' idle
• Engine stalling when coming to a stop or at idle
• Idle speed fluctuating, surging, or 'hunting' up and down
• Vehicle enters 'limp mode' with severely reduced engine power
• Hesitation or stumbling during acceleration
• Whistling or hissing noise from the engine bay, particularly around the throttle body or rear of the supercharger
• In rare, severe cases, unintended acceleration followed by no throttle response

Most Likely Causes

1. Vacuum Leak 🔴 High Probability The M113K engine has numerous vacuum and PCV breather hoses that become brittle and crack from extreme engine bay heat. A hose is often missed during reassembly after other repairs, like valve cover gasket replacement.
   How to confirm: Perform a smoke test. With the engine off, a smoke machine forces low-pressure smoke into the intake system. Smoke will visibly escape from any cracks or loose connections. Pay close attention to the rear of the supercharger and all PCV hose connections.
   Typical fix: Replace the cracked or disconnected vacuum hose(s). A very common culprit is a small aluminum tube with rubber ends (part of the PCV system) that connects to a port on the back of the supercharger, which is easily forgotten or dislodged during a valve cover gasket job. A full PCV breather hose kit is often recommended.
   Est. part cost: $10-$150
2. Dirty or Sticking Electronic Throttle Body 🟡 Medium Probability → Shop Fuel Injection Throttle Body Oil vapor from the PCV (Positive Crankcase Ventilation) system can bake onto the throttle body internals and plate, restricting its fine movements and causing it to stick at or near idle.
   How to confirm: Remove the intake piping leading to the throttle body (located at the rear of the engine, facing the firewall). Visually inspect the throttle plate and bore for a thick layer of black carbon buildup.
   Typical fix: Carefully clean the throttle body bore and plate with a dedicated throttle body cleaner and a soft, lint-free cloth or brush. Do not use harsh tools that could scratch the surfaces. A throttle body reset procedure is required 🎬 See: How to adjust a W211 throttle body after cleaning.
   Est. part cost: $10-$20
3. Failing Electronic Throttle Body ⚪ Low Probability → Shop Fuel Injection Throttle Body The internal electronic motor or position sensor within the throttle body assembly can wear out over time, leading to erratic behavior or complete failure. This is more common on higher mileage vehicles.
   How to confirm: This is difficult to confirm without a bidirectional scan tool (like Mercedes STAR/XENTRY). A technician can command the throttle body to open and close and observe if it responds correctly and smoothly. If it fails to move or acts erratically despite being clean and having good wiring, the unit is likely faulty.
   Typical fix: Replace the entire electronic throttle body assembly. 🎬 Watch: Important tips when replacing your E55 throttle body After installation, an electronic adaptation or 'relearn' procedure using a Mercedes-specific scan tool is necessary.
   Est. part cost: $250-$700

Rare But Worth Checking

• Improperly Installed Aftermarket Throttle Body: → Shop Fuel Injection Throttle Body This code is very common in the enthusiast community after upgrading to a larger (e.g., 82mm) throttle body. The stock ECU cannot control the larger volume of air at idle without a specific software tune. The fix often requires professional tuning or performing a 'default initialization' and idle learning procedure with a Mercedes diagnostic tool.
• Leaking Supercharger Gaskets: A leak from the gasket between the supercharger and the intake manifold, or the bypass flap gasket, can also introduce unmetered air and trigger P0505. This often requires supercharger removal to fix. A smoke test should reveal this type of leak.
• Degraded Engine Wiring Harness: On some older Mercedes models, the insulation on the engine wiring harness can become brittle and crack, leading to short circuits. While less common for this specific code, a short in the throttle body wiring can cause P0505 and other throttle-related faults.

Diagnosis Steps

1. Scan for Codes: Use an OBD-II scanner to confirm P0505 and check for any other related codes like P0106, P0506, or P0507.
2. Visual Inspection: Open the hood and carefully inspect all visible vacuum and PCV breather hoses connected to the intake manifold, airbox, and supercharger for obvious cracks, disconnections, or deterioration. The PCV system hoses are a known failure point.
3. Perform a Smoke Test: This is the most effective way to find hidden vacuum leaks. Connect a smoke machine to a vacuum port (like the brake booster line) and fill the system with smoke. Look for smoke escaping from gaskets, seals, or hoses, especially at the rear of the engine near the firewall.
4. Inspect and Clean Throttle Body: Remove the intake ducting from the throttle body. Check for heavy carbon buildup. If dirty, clean it thoroughly using appropriate cleaner and a soft cloth. Ensure the butterfly valve moves freely.
5. Perform Throttle Body Reset: After cleaning or replacement, a reset procedure is mandatory. A common DIY method is: turn the key to position 2 (all dash lights on) for one minute, turn off for 10 seconds, then start the engine. 🎬 Watch: Step-by-step electronic throttle body idle relearn For best results, especially on modified cars, a reset using a Mercedes-specific scan tool (STAR/XENTRY) is recommended.
6. Check Throttle Body Wiring: Inspect the electrical connector and wiring going to the throttle body for any signs of damage, corrosion, or loose pins.
7. Test Throttle Body Actuator: If the problem persists, a professional with a bidirectional scanner should test the throttle body's electronic actuator to confirm its failure before ordering a replacement part.

Parts You'll Likely Need

• Electronic Throttle Body Assembly (OEM #A1131410125) — If cleaning the throttle body and fixing vacuum leaks do not resolve the code, the internal electronics of the unit itself have likely failed.
  Trusted brands: Bosch, VDO
  OEM price range: $500-$700
  Aftermarket price range: $250-$450
• PCV Breather Hose Kit (OEM #112018KITPRM (URO Parts Kit), individual hoses include 1120180182, 1120180282, 1120180382, 1120180482) — This is the most common cause. The various rubber hoses in the engine bay crack from age and heat, creating unmetered air leaks that the ECU cannot compensate for at idle. Replacing them as a set is wise.
  Trusted brands: Genuine Mercedes-Benz, URO Premium, Cohline
  OEM price range: $80-$150
  Aftermarket price range: $40-$100
• Valve Cover to Supercharger Breather Tube (OEM #A1130100382)

Related Codes That Often Appear With This One

• P0106 — Manifold Absolute Pressure (MAP) Sensor Performance. This code often appears alongside P0505, especially with aftermarket throttle bodies, as the unexpected airflow at idle can lead to MAP sensor readings that don't align with what the ECU expects.
• P0410 — Secondary Air Injection System Malfunction. This system uses vacuum lines to operate. A vacuum leak causing P0505 can also affect the components of the secondary air system, triggering both codes.
• P0506 / P0507 — Idle Air Control System RPM Lower/Higher Than Expected. These are more specific variations of P0505 and often appear when the idle is consistently stuck too low (P0506) or too high (P0507), pointing directly to the same set of root causes.

Platform-Specific Known Issues

• Forgotten Breather Tube After Valve Cover Service: A very frequent cause of P0505 is a small aluminum breather tube with rubber ends near cylinder #8 not being reconnected to its port on the back of the supercharger after a valve cover gasket replacement. The port is difficult to see and easy to miss, leading to a significant vacuum leak. A borescope can help locate the open port without disassembly.
• 82mm Aftermarket Throttle Body Complications: → Shop Fuel Injection Throttle Body Owners who upgrade to a larger 82mm throttle body often trigger P0505 and P0106 codes. The issue stems from the ECU's inability to adapt to the increased airflow at idle. A proper fix requires either a custom ECU tune or a 'default initialization' of the throttle body adaptations using a Mercedes STAR diagnostic tool. Simply clearing codes is usually not a permanent solution.

Mechanic-Grade Diagnostic Values

• Throttle Position Sensor (TPS) percentage at idle, in Drive — expected: Approximately 3.2% after a successful reset.. Failure: Readings significantly different from the target, or fluctuating erratically, can indicate a failed adaptation or faulty sensor.
• Throttle Body Potentiometer Voltages (Key On, Engine Off) — expected: With the throttle body connected, the two potentiometer signals should have an inverse relationship and their sum should be constant. With the connector unplugged, both signal circuits at the harness should read near 0V.. Failure: A reading of a constant 5V on one of the potentiometer signal circuits even when the throttle body is unplugged suggests a short to the 5V reference line in the engine wiring harness.
• Engine Ground Strap Voltage Drop Test — expected: Less than 0.5 Volts.. Failure: A reading of more than 0.5V when measuring between the negative battery terminal and a clean point on the engine block while cranking indicates high resistance from a corroded or faulty ground strap.

Scan Tool Commands That Help

• Mercedes STAR/XENTRY: Default Initialization (within ME Control Unit adaptations) — This is the most reliable way to reset all throttle body adaptations. It should be performed after cleaning or replacing the throttle body, or when troubleshooting idle issues, especially with aftermarket parts.
• Mercedes STAR/XENTRY: Correction Programming (Idle Speed) — Used to manually raise or lower the target idle speed in Park and Neutral. This is often necessary to resolve a lumpy idle after installing a larger, 82mm throttle body, where an increase of ~50 RPM may be required.
• Mercedes STAR/XENTRY: Mixture Self-Tuning Adaptation — This screen shows the status of idle and partial load fuel trim adaptations. If the idle range shows 'not completed', the tool may provide a guided driving procedure (e.g., hold 3rd gear above 2100 RPM) to force the ECU to learn the new values.

Wiring & Ground Locations

• W11 — Combustion engine ground point. This is the main ground strap from the engine block to the chassis.. A corroded or loose main engine ground can cause a host of erratic electronic issues, including incorrect sensor readings and faulty actuator behavior, which can trigger a P0505.
• Z6/50 — Ground connector sleeve located near the left headlamp unit (W9).. This is a common grounding point for various electronic components in the engine bay. A fault here could affect the throttle body's circuit.
• Throttle Body Connector — At the rear of the engine, connecting to the electronic throttle body which faces the firewall.. The connector itself can be a point of failure. The wiring loom can be very stiff, requiring care during removal to avoid damage. On early 2003 models, the harness may be epoxied into the throttle body, making it non-removable.

Real Owner Repair Stories

• MBWorld.org user post (W211 E55 AMG) — P0505 Check Engine Light immediately after performing a valve cover gasket replacement.
  ❌ Tried (didn't work) Visually checking all the main, obvious breather hoses multiple times.
  ✅ What actually fixed it The user had forgotten to reconnect a small aluminum breather tube to its port on the back of the supercharger, near cylinder #8. They found the open port using a borescope and reattaching the tube resolved the code.
• MBWorld.org user post (E55 AMG with 82mm aftermarket throttle body) — P0505 code with a very lumpy idle, hiccup when shifting, and a forward slug when letting off wide-open throttle.
  ❌ Tried (didn't work) Performing a smoke test, which showed no vacuum leaks.
  ✅ What actually fixed it Using a Mercedes STAR diagnostic tool, the user performed a 'default initialization' to reset adaptations, followed by using 'correction programming' to raise the idle speed by 50 RPM in Park and Neutral. This combination of software adjustments fixed all symptoms.
• MBWorld.org user post (2006 CLS55 AMG (shares M113K engine)) — Recurring P0505 code that kept returning after being reset.
  ❌ Tried (didn't work) Simple OBD-II code reset., Checking for visible hose leaks.
  ✅ What actually fixed it A specific, multi-step throttle body reset procedure worked: 1) Key off, unplug throttle body connector. 2) Key on for several minutes. 3) Key off, plug throttle body back in. 4) Press gas pedal down once, then turn key to position 2. 5) With key on, press pedal down once more and wait 2 minutes for the TPS to reset. This resulted in a stable 3.2% TPS reading in drive.

"I Checked Everything" — The Actual Cause

• A very common scenario for this code on modified E55s is when an aftermarket, larger-bore (e.g., 82mm) throttle body is installed. A smoke test will come back clean because there are no unintended leaks, but the P0505 code is triggered because the Engine Control Unit's factory software cannot adapt to the significantly larger volume of air at idle. The actual fix is not finding a leak, but using a Mercedes STAR/XENTRY tool to perform a 'Default Initialization' of the throttle body and often manually increasing the target idle RPM.

OEM Part Supersession History

• A1131410125 (Mercedes-Benz) → 0 280 750 017 (Bosch) — This is the OEM equivalent part number from the original manufacturer, Bosch.
  Heads up: While a direct replacement, ensure you are buying from a reputable source like Bosch, VDO, or Genuine Mercedes. Avoid unbranded clones which may have electronic incompatibilities.

Model Year Variations Within This Range

• Early 2003: The wiring harness connector may be permanently epoxied into the electronic throttle body assembly on some early 2003 models. On later models, the connector is removable. This is critical to know if planning to replace the throttle body, as an early model may require more extensive wiring work or a specific harness.

Diagnostic Flowchart

Other Known Issues on This Vehicle

Issues unrelated to this code that are worth knowing about as an owner of this generation:

• Sensotronic Brake Control (SBC) Pump Failure 🔴 High — The pump has a predefined service life based on actuations. Failure is inevitable. Onset is typically after 10-15 years. An extended warranty to 25 years was issued in the US. (Ref: NHTSA Campaign ID: 04V296000, 05V133000 (Recalls for hydraulic unit))
• Airmatic Suspension Leaks and Compressor Failure 🔴 High — Very common. Air struts/bags develop leaks, and the compressor burns out from running constantly to compensate. Often occurs around 80,000-120,000 miles.
• Leaking Valve Cover Gaskets and PCV Hoses 🟠 Medium → Shop PCV Valve Hose — Extremely common. Gaskets and hoses become brittle from heat, causing oil leaks onto exhaust manifolds and vacuum leaks. Often requires attention every 50,000-70,000 miles.
• Supercharger Clutch Pulley Bearing Failure 🟠 Medium — The bearing in the clutched supercharger pulley wears out, causing a rattling or grinding noise at idle when the supercharger is disengaged. Typically occurs at higher mileage (100,000+ miles).
• Leaking Fuel Sending Unit Gaskets 🔴 High — The seals on the fuel sending units under the rear seat can degrade, causing a strong fuel smell inside and outside the car. This is a fire hazard. Mercedes issued an extended warranty for this issue. (Ref: Extended Warranty Coverage)
• Failed Intercooler Pump 🟠 Medium — The stock pump for the supercharger's intercooler is prone to failure, causing the ECU to pull timing due to high intake air temperatures (heat soak), resulting in significant power loss. Many owners upgrade to the more robust Bosch '010' pump.

Used vs. New Parts: Buying Guide for This Vehicle

When a used part is the smart pick: A used OEM electronic throttle body (Bosch/Mercedes) can be a viable, cost-saving option if it comes from a lower-mileage, reputable salvage source. It is a relatively durable component.

Donor-vehicle mileage cap: roughly under 100000 miles for the part to have meaningful remaining life.

What to inspect on the donor part:

• Check for a clean throttle plate and bore, free of heavy carbon or oil sludge.
• Inspect the electrical connector pins for any signs of corrosion, moisture, or physical damage.
• Ensure the part number is an exact match (A1131410125 or Bosch 0280750017).
• If possible, confirm the donor vehicle did not have extensive engine damage.

OEM-only on this vehicle (don't cheap out):

• Electronic Throttle Body: While Bosch is the OEM, it is strongly recommended to stick to Genuine Mercedes or Bosch branded parts. Unbranded aftermarket electronic components are a significant gamble for compatibility and longevity.
• PCV/Breather Hoses: These should always be purchased new. They fail due to age, heat, and material degradation, so a used hose has no meaningful life left.

Aftermarket brands forum-validated for this vehicle:

• Bosch (for Throttle Body - OEM)
• VDO (for Throttle Body)
• URO Premium (for PCV hoses)
• Cohline (for PCV hoses)

Brands owners have reported issues with on this vehicle:

• Unbranded, no-name electronic throttle bodies from online marketplaces (e.g., eBay, Amazon) are frequently reported as dead-on-arrival or failing shortly after installation.

Real Owner Stories

Aggregated from forums and TSBs cited above. Mileages and costs reflect what owners reported in those sources.

Related OBD-II Codes

• P0106 — Manifold Absolute Pressure sensor code that frequently appears alongside P0505 when upgrading to an 82mm throttle body or when vacuum leaks are present.
• P0506 — Indicates idle control system RPM is lower than expected; often diagnosed alongside P0505 using the same steps.
• P0507 — Indicates idle control system RPM is higher than expected; shares common causes like vacuum leaks or throttle body carbon buildup.

Frequently Asked Questions