P1605: Rough idling (Toyota)

Rough idling

Severity: Inspect soon

On Toyota, P1605 means the idle sagged or wavered badly enough to nearly stall. The engine usually keeps running, but it can stall at a stop, so get it diagnosed soon.

It gets more serious if:

Stop: The engine stalls or loses power while driving. Owners report P1605 with stalls at stoplights and loss of speed on the highway. Get off the road safely and have it checked or towed before driving again.

Source: how we rate severity

P1605 in short

On Toyota vehicles, P1605 means rough idling: engine speed at idle dropped or wavered beyond what the engine computer could correct, close to stalling. Air leaks, dirty throttle or fuel problems are typical; flex-fuel V8s can misjudge fuel alcohol. Get it checked soon, and stop driving if the engine stalls on the road.

P1605 is a manufacturer code, so its meaning depends on the make. This page covers Toyota, where it is called Rough Idling.

At idle the engine computer holds a target speed by adjusting the throttle, and it watches crankshaft sensor pulses to see how evenly the engine turns. If idle speed drops below the set speed, or the correction it must apply grows too large, it stores P1605.

The code describes a symptom, so the cause can be anything that upsets idle: an air leak, a dirty throttle, weak fuel delivery, misfire, or a wrong learned fuel mixture. It is often stored with P1603 (engine stall history) and P1604 (startability).

Pick your make

P1605 is a manufacturer-specific code: its meaning depends on who built the car.

Other makes: meaning not verified. Check with a scan tool that reads manufacturer definitions.

P1605 on Toyota

Most likely causes

Cause 1: Air leak, dirty throttle or other idle air and fuel problem
Common general cause
Why it ranks here
TAT describes P1605 as idle speed or idle correction going out of limits, which is what intake leaks, a dirty throttle and weak fuel delivery do. Toyota bulletins pair it with lean and rich fuel trim codes.
What would change this
Rises with lean fuel trims, a hissing leak, or a dirty throttle plate; falls if trims and idle airflow are normal.
How to test the electronic throttle body
Cause 2: Wrong learned fuel alcohol content (Toyota 3UR-FBE flex fuel)
Supported by vehicle data
Why it ranks here
Toyota T-SB-0058-13 links P1605 and P1604 with fuel trim codes on 2012-2015 Sequoia and Tundra flex-fuel V8s to an alcohol estimate too high for the fuel used; ALLDATA describes fixing a Sequoia by resetting the learned value.
What would change this
Applies to the listed trucks; rises if the alcohol estimate is high while running regular fuel.
How to test the engine control module (ecm / pcm)
Cause 3: Water and corrosion in the engine computer connector (2014-2018 Highlander)
Supported by vehicle data
Why it ranks here
Toyota T-SB-0072-19 lists P1605 with P1603, P1604, low fuel pressure and misfire codes from water in the A43 engine computer connector, traced to aftermarket windshield de-icer harnesses.
What would change this
Applies to the listed Highlanders; rises if an aftermarket de-icer windshield is fitted or the connector shows corrosion.
How to test the engine control module (ecm / pcm)
Cause 4: Carbon build-up on the pistons (2012-2013 Scion iQ)
Supported by vehicle data
Why it ranks here
Toyota's Scion bulletin S-SB-0008-16 ties rough idle, idle dips and P1603-P1605 on the iQ to heavy carbon on the piston heads.
What would change this
Applies to the Scion iQ.

What to test first

Start at step 1. Each result tells you what it means and where to go next.

Step 1: Read every stored code and the P1605 freeze-frame data, noting fuel trims, coolant temperature and engine speed when it set.
Low riskTool: scan tool
- IfLean fuel trims or a lean code
  It meansLook for an air leak or weak fuel delivery.
  Points to: Air leak, dirty throttle or other idle air and fuel problem
- IfFlex-fuel Sequoia or Tundra with fuel trim codes
  It meansToyota's alcohol-learning bulletin likely applies.
  Points to: Wrong learned fuel alcohol content (Toyota 3UR-FBE flex fuel)
- IfNothing stands out
  It meansHave a shop check bulletins and the engine computer connector.
  Go to step 2
Step 2: Have a shop check for bulletins on the VIN, smoke-test the intake for leaks, inspect the throttle and the engine computer connector, and measure fuel pressure.
Best left to a shopTool: scan tool, smoke machine, fuel pressure gauge
Warning:Fuel pressure testing opens a pressurised fuel system, which is a fire risk. Leave it to a shop.
- IfIntake leak or a dirty throttle
  It meansRepair the leak or clean the throttle and relearn idle.
  Points to: Air leak, dirty throttle or other idle air and fuel problem
- IfCorrosion at the engine computer connector
  It meansRepair the connector and seal the source of water.
  Points to: Water and corrosion in the engine computer connector (2014-2018 Highlander)
- IfA carbon build-up bulletin applies
  It meansFollow the bulletin repair.
  Points to: Carbon build-up on the pistons (2012-2013 Scion iQ)

Symptoms

Rough, shaky or dipping idle
Engine nearly stalls at a stop
Check engine light, often with P1603 or P1604
Engine stalls or loses speed while driving Gets more serious: stop and have it checked.

How to fix

Step 1: Write down every stored code and the freeze-frame data before clearing anything.
Step 2: Ask a dealer whether a bulletin or software update covers rough idle on your VIN.

Parts and cost context

Electronic throttle body:Cleaning a dirty throttle can help, but Toyota needs an idle relearn afterwards or idle speed may be wrong. Labour varies; get a quote.

Labor varies by vehicle and shop; get a quote before you approve a repair.

Compare used part prices on PartPriceGuide

On your vehicle

Toyota

2012–2015 · Sequoia · 3UR-FBE
Supported by vehicle data
On the flex-fuel 3UR-FBE, a wrong learned alcohol content can set P1605 with P1604 and fuel trim codes (T-SB-0058-13).
2012–2015 · Tundra · 3UR-FBE
Supported by vehicle data
Same flex-fuel alcohol-learning bulletin as the Sequoia (T-SB-0058-13).
2014–2018 · Highlander
Supported by vehicle data
Check the engine computer connector for water and corrosion if an aftermarket windshield with a de-icer is fitted (T-SB-0072-19).
2015–2018 · RAV4
Possible — needs testing
Owners report stalls at stoplights and in traffic with P1605 stored. Owner reports only.

T-SB-0058-13Status unverified
Toyota T-SB-0058-13 (revised): 3UR-FBE MIL with P1604, P1605 and fuel trim codes
2012–2015sequoia, tundra · 3UR-FBE · issued December 11, 2014 · NHTSA 10132332
Some 2012-2015 Sequoia and Tundra trucks with the 3UR-FBE engine may set lean or rich codes, P1604 (startability) or P1605 (rough idling) after starting because the alcohol density estimate is high for the fuel actually used.
We couldn't confirm whether this bulletin is still current, so it doesn't count as confirmation.
T-SB-0072-19Status unverified
Toyota T-SB-0072-19: drivability and P1603-P1605 from ECM connector corrosion
2014–2018highlander · issued June 24, 2019 · NHTSA 10162756
Water and corrosion in the A43 engine computer connector, from poorly sealed aftermarket windshield de-icer harnesses, can cause drivability problems, low fuel pressure and codes including P1603, P1604 and P1605.
We couldn't confirm whether this bulletin is still current, so it doesn't count as confirmation.

All Engine computer (ECM/PCM) codes

P1605 questions

Is P1605 the same as a misfire code?

No. On Toyota it means idle speed sagged or wavered too much. A misfire can cause that, but so can air leaks or fuel problems, so check the other stored codes.

Sources and review

TAT Scandata: P1605 (Toyota) Rough Idling — Technical Assistance Team (TAT), Australia
Toyota T-SB-0058-13: 3UR-FBE MIL with P1604 Startability and P1605 Rough Idle — Toyota Motor Sales / NHTSA , April 26, 2013
NHTSA complaint ODI 10928436: 2015 Toyota RAV4 stalls at stops with P1605 — NHTSA Office of Defects Investigation (complaint file) , November 28, 2016
NHTSA complaint ODI 11725081: 2020 Toyota Tacoma idles erratically and loses speed on the highway, P1605 — NHTSA Office of Defects Investigation (complaint file) , March 17, 2026
NHTSA complaint ODI 11205338: 2018 Toyota RAV4, dealer readout P1603 engine stall, P1605 rough idling — NHTSA Office of Defects Investigation (complaint file) , May 2, 2019
ALLDATA Tech Tip: Sequoia extended cranking, DTCs P1601-P1605 — ALLDATA
Toyota T-SB-0072-19: Highlander ECM connector corrosion, P1603/P1604/P1605 — Toyota Motor Sales / NHTSA , June 24, 2019
Scion S-SB-0008-16: iQ rough idle, idle dip, P1603/P1604/P1605 — Toyota Motor Sales / NHTSA , October 20, 2016
NHTSA complaint ODI 11139909: 2017 Toyota Corolla stalled while driving, P1603 and P1605 — NHTSA Office of Defects Investigation (complaint file) , October 12, 2018

Last reviewed: October 15, 2026
Written by: Saurabh K.
Editorial review: Pending
Technician review: Technician review pending

This page explains what a code usually means; it is not a substitute for a professional inspection. How we source and review · Report an error

Severity: Inspect soon

Pick your make

Most likely causes

Cause 1: Air leak, dirty throttle or other idle air and fuel problem

Cause 2: Wrong learned fuel alcohol content (Toyota 3UR-FBE flex fuel)

Cause 3: Water and corrosion in the engine computer connector (2014-2018 Highlander)

Cause 4: Carbon build-up on the pistons (2012-2013 Scion iQ)

What to test first

Step 1: Read every stored code and the P1605 freeze-frame data, noting fuel trims, coolant temperature and engine speed when it set.

Step 2: Have a shop check for bulletins on the VIN, smoke-test the intake for leaks, inspect the throttle and the engine computer connector, and measure fuel pressure.