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Engine troubles can be frustrating, especially when they lead to rough idling. One common diagnostic trouble code that Toyota owners may encounter is P1605.

In this comprehensive guide, we will delve into the meaning, severity, symptoms, causes, diagnosis, and repair steps related to P1605 Toyota.

By the end, you’ll have a better understanding of this code and be equipped to address it effectively.

P1605 Toyota: A Quick Overview

Let’s start with a quick overview of P1605 for Toyota. This code is associated with engine rough idling, and its severity level is considered medium.

While you may be tempted to continue driving, it is advisable not to ignore this code. Ignoring it can lead to decreased engine performance, rough idling, and potential stalling. The estimated repair cost for addressing P1605 Toyota ranges from $200 to $500.

Meaning of P1605 Toyota – Engine Rough Idling

The P1605 Toyota code is triggered when the engine speed drops below the set speed after 5 seconds or more following engine startup.

The onboard computer (ECM) detects this decrease using a one-trip detection logic. P1605 is typically associated with the engine control and Sequential Fuel Injection (SFI) systems.

These systems work together to regulate the engine’s performance, fuel injection timing, and other parameters for smooth operation.

Severity of P1605 Toyota

When it comes to severity, P1605 Toyota is considered to have a moderate level of severity.

While it may not pose an immediate safety risk, ignoring or neglecting this code can lead to various issues such as rough idling, decreased engine performance, and potential stalling.

To ensure the longevity and optimal performance of your Toyota vehicle, it is crucial to address the P1605 code promptly.

Read also >> Knock Sensor Circuit Malfunction (Resolving the P0325 Code)

Symptoms of P1605 Toyota

The P1605 Toyota code can manifest in several symptoms, indicating an issue with engine speed or idle control. Common symptoms include:

  1. Illuminated check engine light
  2. Rough idle
  3. Slow acceleration
  4. Engine vibration leading to engine shutdown
  5. Engine shutdown without vibration
  6. Engine starts with accelerator pedal depressed
  7. Rough idling after engine startup

If you are experiencing any of these symptoms, there is a possibility that your vehicle is affected by the P1605 code.

Causes of P1605 Toyota

The P1605 Toyota code can have various underlying causes that trigger the engine speed drop. These causes can be categorized based on the system or component involved. Here are some potential causes:

Intake System

  • Air leak in the intake system

Emission Control System

  • Brake booster assembly
  • Purge VSV
  • Sensor and sensor circuit
  • Power supply circuit
  • Engine coolant temperature sensor
  • Mass air flow meter subassembly

Fuel System

  • Fuel pump control circuit
  • Fuel pump
  • Fuel line
  • PCV valve and hose

Engine Control System

  • Ignition system
  • Camshaft timing oil control valve assembly
  • Knock control sensor

Other Systems and Components

  • Wire harness or connector
  • Thermostat
  • Park/Neutral Position switch assembly
  • Air Conditioning system
  • Electrical load signal system
  • Power Steering system
  • Charging system
  • Automatic Transaxle system
  • Immobilizer system
  • ECM

Diagnosing and Repairing P1605 Toyota {#diagnosis}

To accurately diagnose and repair the P1605 Toyota code, you may need various tools and parts, including:

  • Diagnostic scanner
  • Multimeter
  • Vacuum gauge
  • Smoke machine
  • Intake gaskets
  • Sensor replacement parts
  • Ignition components
  • Fuel system components

The diagnostic and repair procedure for P1605 Toyota involves two cases based on the engine speed decrease. Let’s explore each case:

Case 1: Engine Speed Decreased Slowly

  1. Inspect ignition system components, such as spark plugs, power supply circuit, and ignition coil assembly. Look for any faults or malfunctions and repair or replace as necessary.
  2. Perform an air-fuel ratio check by inspecting the intake system for air leaks. Check connections and the brake booster assembly. Repair any leaks found.
  3. Test air-fuel ratio related sensors, including the mass air flow meter, air-fuel ratio sensor, thermostat sensor, and engine coolant temperature sensor. Repair or replace malfunctioning sensors indicating richness or leanness beyond normal limits.
  4. Verify the fuel supply system, including the fuel pump control circuit, purge VSV system, fuel lines, and ECM. Repair or replace any damaged components.
  5. If issues persist, check the intake air volume for sufficiency. Inspect the air passage for obstructions and excessive valve overlap in the camshaft timing oil control valve assembly. Repair or replace faulty components if necessary.
  6. Clean the intake manifold and throttle body if required.
  7. Check the vacuum system for leaks and repair any detected issues.
  8. Finally, verify that the ignition timing is operating properly and adjust if necessary.

Case 2: Engine Speed Decreased Rapidly

  1. Inspect the ignition and injection system for any electrical system malfunctions.
  2. Look for signs of power interruption or temporary power cuts.
  3. Examine the power supply circuit to the fuel injector and ignition coil assemblies. Repair or replace any faulty components in the electrical system.
  4. Verify external components that may be causing an increased load on the engine, such as the power steering or air conditioning system.
  5. Address any external part malfunctions by repairing or replacing the faulty components and ensuring proper functioning of external systems that can impact engine load.

DIY Repair Level and Estimated Costs

The repair procedure for the P1605 Toyota code is classified as advanced. It involves checking and inspecting multiple components within the engine system, requiring a good understanding of engine systems, diagnostic tools, and troubleshooting techniques.

If you are unsure or uncomfortable with the procedure, it is highly recommended to seek assistance from an experienced mechanic or automotive professional who can accurately diagnose and resolve the issue.

Estimated costs for main repair tasks associated with P1605 Toyota can vary based on factors such as location, vehicle model, and labor rates. Here are some estimated costs:

  • Wiring repairs: $50 – $150
  • Vacuum system leak repair: $50 – $150
  • Air-fuel ratio sensor replacement: $50 – $200
  • Mass air flow meter replacement: $100 – $300
  • Thermostat sensor replacement: $50 – $150
  • Engine coolant temperature sensor replacement: $50 – $150
  • Ignition component replacement: $100 – $300
  • Fuel system component replacement: $100 – $500

Remember, these are estimated costs, and it is advisable to consult with a professional for accurate cost estimates.


In conclusion, the P1605 Toyota code indicates engine rough idling and should not be ignored. It is important to address this code promptly to ensure the optimal performance and reliability of your Toyota vehicle.

By following the diagnostic and repair steps outlined in this guide, you can effectively diagnose and resolve the underlying issues causing the P1605 code.

If you feel uncertain or uncomfortable with the repair process, don’t hesitate to seek assistance from a qualified mechanic. Take action today and get your Toyota back to smooth idling and optimal performance.

Steve P.

Steve is an automotive technician, technical writer, and Managing Editor. He has held a lifelong passion for cars, with a particular interest in cars like the Buick Riviera. Steve is based in Boise, Idaho.

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