Toyota P1605 Code: Rough Idling (Older Engines: Knock Control CPU)
On newer Toyotas the idle sagged below about 400 rpm; on some older ones it flags the ECM's knock processor.
Unverified: this manufacturer-specific definition comes from a single source or varies by model. Confirm it with a brand-specific scan tool.
At a glance
Severity: Low
- Meaning
- Rough Idling (Older Engines: Knock Control CPU)
- System
- Speed & Idle Control · Powertrain
- Safe to drive?
- With caution
- Typical repair cost
- $40–$1,500
- DIY difficulty
- Moderate
P1605 has two meanings depending on the ECM generation, so this page is marked unverified for your exact car. On Toyotas from roughly the mid-2000s onward it is Rough Idling: an event record stored when, at least five seconds after start-up, engine speed falls to about 400 rpm or less. It is the near-stall companion of P1603, and the ECM stores it first, so a car that sagged and recovered shows only P1605, while one that died shows both. On some late-1990s and early-2000s Toyota engines, several scan tools list P1605 as Knock Control CPU Malfunction, an internal fault of the knock-processing circuit in the ECM itself. Check which definition your scan tool and model year use before you start: one points to idle air, vacuum leaks and misfires, the other to the engine computer.
Can I keep driving? The car can usually be driven, but a sagging idle can turn into a stall at junctions. Keep a little throttle when stopping with heavy loads switched on until the cause is found.
How to read P1605
- P Powertrain
- 1 Manufacturer-specific
- 6 Computer & outputs
- 05 Specific fault
Symptoms
- Idle speed dips hard when the A/C, fans or headlights switch on
- Engine shudders and nearly stalls when rolling to a stop
- Idle hunting up and down after a battery change or throttle cleaning
- Code found during a scan with no warning light on (newer models)
- Knock-related codes or poor power on older models using the ECM definition
Possible causes
Ranked by how often each cause is behind this code (editorial estimate).
- Carbon build-up in the throttle body restricting idle air
- Vacuum leak or contaminated MAF sensor under-reading at idle
- Idle learning lost after battery disconnection or throttle cleaning
- Idle control unable to cope with sudden loads such as the A/C compressor or cooling fan
- Misfire at idle from a weak coil or worn spark plug
- EGR or purge valve leaking at idle on engines that have them
- Older ECM definition: knock control processor inside the ECM failed
Diagnosis step by step
Confirm which P1605 you have
Note the model year and what a Toyota-capable scan tool calls the code. Rough Idling sends you to the idle and air path; Knock Control CPU on an older model means the ECM itself, and the steps below about idle air don't apply.
Tools: OBD-II scanner
Study the freeze frame
Look at target versus actual idle speed, vehicle speed, throttle angle and whether the A/C or electrical loads were on when the dip happened. A dip only with loads on points to idle control reserve; a dip at any time points to air leaks or misfire.
Tools: OBD-II scanner
Recreate the dip with loads
With the engine warm and idling, switch the A/C, rear defroster and headlights on and off one by one. A healthy engine drops a few dozen rpm and recovers in a second; a drop of several hundred rpm or a near-stall confirms the complaint.
Tools: OBD-II scanner
Clean the throttle body and relearn idle
With the key off, remove the intake boot and clean the carbon from the bore and the plate edge with throttle cleaner on a rag, without forcing an electronic throttle open by hand. Then complete the idle learning your model requires; some Toyotas learn at idle by themselves, others need the dealer scan tool.
Tools: Screwdriver, Soft brush
Check fuel trims and hunt leaks
Compare long-term fuel trim at idle and at about 2,500 rpm. High positive trim at idle that normalizes with rpm is the signature of a vacuum leak; smoke-test the intake, PCV hose and brake booster line to find it.
Tools: Smoke machine, OBD-II scanner
Check for idle misfire
Read the misfire counters per cylinder at warm idle. A single cylinder with a rising count points to its coil or plug; swap the coil to a different cylinder to confirm before buying parts.
Tools: OBD-II scanner, Spark plug socket
Where the parts are
- 1MAF sensor
- 2Turbocharger
- 3Throttle body
- 4Intake manifold
- 5Fuel injector
- 6EGR valve
- 7Fuel pump
- 8Thermostat
Repair options & costs
| Repair | DIY? | Estimated cost |
|---|---|---|
| Clean throttle body and perform idle relearn | Yes | $80–$200 |
| Replace PCV valve or cracked vacuum hose | Yes | $40–$200 |
| Replace ignition coil or spark plugs | Yes | $100–$320 |
| Clean or replace EGR valve | No | $250–$700 |
| Replace ECM (older knock control CPU definition) | No | $600–$1,500 |
Rough US estimates for parts and labor; shop rates vary by region. Not a quote.
Emissions / smog test impact
Depends
On newer Toyotas P1605 is a history record usually stored without the check engine light, so the smog check depends on the light and monitor status. On older engines using the ECM definition the light is typically on, which fails the test.
Frequently asked questions
My scanner says Knock Control CPU but the car only idles rough. Which is right?
Many generic scan tools show the older definition for every Toyota. On a newer model the rough-idle meaning is almost certainly the one that applies; a Toyota-capable tool shows the correct text for your ECM.
P1605 appeared right after I replaced the battery. Why?
Disconnecting the battery erases the learned idle values. Until the ECM relearns them, idle can sag low enough to set P1605, especially with the A/C on.
Why do I have P1605 without P1603?
The engine sagged below about 400 rpm but recovered instead of stalling. If it had died completely, P1603 would have been stored as well.
Can cleaning the throttle body fix P1605?
Often yes, on engines that have built up carbon behind the throttle plate. Clean it with the key off and let the ECM relearn idle afterwards, otherwise the idle may run high or hunt.
Sources
- Manufacturer public service information (cross-checked)
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