P1508

Code

INFINITI P — Powertrain

IAC Valve Circuit Failure

Brand: INFINITI

Type: P — Powertrain

Views: UK: 30 EN: 48 RU: 31

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Page language: EN

Causes

Open or shorted wiring to the IAC valve
Corroded, loose, or damaged connector pins
Faulty IAC valve (stuck, burned coils, failed stepper/motor)
Poor ground or blown fuse for IAC circuit
Carbon/soot build-up or mechanical sticking in throttle body/IAC
ECM control/output driver fault

Symptoms

Check Engine Light (CEL) illuminated (P1508 stored)
Unstable or fluctuating idle RPM
High idle or idle too low, possibly stalling
Engine stumbles or stalls at idle or when coming to a stop
Long crank or rough idle after starting
Difficulty maintaining set idle under load (A/C on)

What to check

Read and record freeze-frame and related codes with a scan tool
Visually inspect IAC connector and wiring harness for damage, corrosion, pin push-out, or melted insulation
Verify fuses and power/ground circuits for the IAC and ECM
Scan for live data: commanded IAC position/PWM and actual feedback (if available)
With ignition ON (engine OFF) confirm IAC supply voltage (battery voltage) and reference signal presence
Measure IAC coil resistance per OEM spec or bench-test actuator function

Signal parameters

Supply power: typically ignition-switched battery voltage (~12 V) present at IAC power pin with key ON (verify per model)
Control signal: often a 0–5 V reference or PWM duty cycle from ECM (measure with scope/multimeter)
Ground: low-resistance path to chassis/ECM ground required
Actuator resistance: varies by design—consult Infiniti service manual for specific coil/stepper ohms and pinout
Expected behavior: when commanded by scan tool, IAC should move/produce a measurable change in PWM or duty cycle and corresponding idle RPM change

Diagnostic algorithm

Connect a scan tool, record freeze-frame, and note related codes (e.g., idle or throttle codes).
Perform a visual inspection of IAC harness and connector; repair any obvious damage. Check and replace blown fuses.
With ignition ON (engine OFF) verify battery voltage at the IAC power terminal and verify good ground at the connector. Repair as needed.
Back-probe control pin while commanding IAC via scan tool; verify control signal (voltage or PWM) and that ECM is issuing commands. If no command, suspect ECM or related inputs (throttle position sensor, coolant temp).
Measure IAC coil/stepper resistance and compare to service specification. If out of range, replace IAC. If within range, perform dynamic test: command IAC and observe engine RPM response or use oscilloscope to verify waveform.
Inspect and clean throttle body and IAC pintle/port for carbon that can mechanically bind movement; re-test after cleaning.
If wiring and actuator test good but no proper response, perform wiggle test on harness while monitoring live data. Repair wiring/connector faults and re-test.
If wiring/actuator confirmed good and ECM does not command or drive outputs properly, consider ECM diagnosis/reprogramming/repair per manufacturer procedures.
Clear codes and perform road/idle test to confirm repair. Document results and re-check for intermittent faults.

Likely causes

Wiring/connectors (damage or corrosion) — most common
IAC valve electrical failure or mechanical seizure
Blown fuse or poor ground
Throttle body contamination preventing movement
ECM driver failure (less common)

Fault status

⚠️ Status

ECM detected IAC valve circuit fault: open/short/out-of-range or no response from idle control actuator. Drive prevented from achieving commanded idle control.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 0.5–2.0 hours

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