P1605

Code

SCION P — Powertrain

Rough idle / ECM processing fault

Brand: SCION

Type: P — Powertrain

Views: UK: 4 EN: 9 RU: 1

AI status

Completed

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Page language: EN

Causes

Low or unstable battery/charging voltage
Poor or corroded ECM power/ground connections
Damaged ECM connector or wiring harness (opens/shorts/intermittent)
Faulty ECM/PCM internal hardware or software error
Communication errors on CAN/serial data bus or between modules
Related sensor faults (throttle position, MAF, MAP, coolant temp, IAC/idle control) causing abnormal ECM processing

Symptoms

Rough or unstable idle
Engine stalls at idle or shortly after start
Check Engine Light (MIL) illuminated with stored P1605
Possible limp-home mode or reduced performance
Multiple related trouble codes (sensor, communication or misfire codes) may be present

What to check

Retrieve freeze-frame and full list of stored codes and pending codes with a professional scanner
Check battery resting voltage and voltage with engine running (normal 12.5–14.5 V)
Inspect battery terminals, main fuses, ECM power supply and ground connections for corrosion, tightness and continuity
Visually inspect ECM connector and wiring harness for damage, water intrusion, corrosion or pin push-out
Scan live data: engine RPM, TPS, MAF/VAF, MAP, IAT, coolant temp, IAC/idle duty and compare against expected values
Check CAN/serial bus communication status for errors or bus-off conditions

Signal parameters

Battery voltage (key on): approx. 12.2–12.8 V; running: 13.5–14.8 V
Cranking voltage: should not drop below ~9.5 V under load (varies by vehicle)
Idle speed (engine off accessories): ~600–900 RPM (engine warm; manufacturer-specific)
TPS at idle: typically 0.5–1.5° or 0.2–1.0 V depending on sensor type
MAF airflow at idle: small steady value (manufacturer-specific)
MAP at idle: near atmospheric minus manifold vacuum (typical ~30–60 kPa absolute depending on altitude)

Diagnostic algorithm

Read and record all stored and pending DTCs and freeze-frame data with a capable scan tool; do not replace components based on P1605 alone.
Clear codes and attempt to re-create the condition with live-data monitoring to see which inputs deviate when P1605 sets.
Check battery state and charging system: measure resting and running voltage. Repair any low-voltage/charging faults and retest.
Inspect and test main power and ground circuits to the ECM: clean terminals, tighten connections, verify continuity to chassis and battery.
Inspect ECM connector and wiring visually and by backprobing where safe: look for bent pins, corrosion, chafing, or intermittent shorts/opens. Wiggle test harness while watching live data.
Verify communication on data buses (CAN/serial) and check for other modules reporting errors. Repair bus issues before replacing ECM.
Evaluate sensors that directly affect idle control (TPS, MAF/MAP, IAT, coolant temp, idle control actuator); confirm plausible signal ranges and correct sensor operation.
Check for intake vacuum leaks, throttle body contamination, or mechanical issues that cause unstable idle. Repair as needed and retest.
If wiring, grounds, power and sensors are confirmed good and the fault persists, consider ECM software update/reprogramming per dealer bulletin. If reflash does not correct the issue and diagnostics point to internal ECM faults, replace ECM with proper programming and immobilizer/anti-theft matching.
After any repair, clear codes and perform drive cycle verifying the code does not return and idle is stable.

Likely causes

Weak battery or poor battery terminal connection causing voltage drops
Corroded/loose ECM ground or power feed
Intermittent wiring to throttle body/idle control or key sensors (TPS, MAF, MAP)
ECM software glitch or internal failure (less common)

Fault status

⚠️ Status

P1605 — ECM processing fault: engine control module detected an internal processing or input condition preventing normal idle control. May cause rough idle, stalling or trigger limp mode. Inspect power/ground, wiring, communications and related sensors before replacing the ECM.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 1.0-3.0 hours

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P1605 — Rough idle / ECM processing fault