Code
P1491
Generic
P — Powertrain
Secondary Switch Solenoid Circuit Malfunction
Views:
UK: 51
EN: 45
RU: 31
AI status
Completed
Completed
100%
Causes
- Open or short in solenoid wiring (broken wire, chafing, pinched harness)
- Corroded, loose or damaged connector/pins at solenoid or PCM
- Failed solenoid (coil open, shorted, or internally intermittent)
- Blown fuse or faulty relay supplying the solenoid circuit
- Poor or missing ground
- PCM/ECM driver fault or internal software error
Symptoms
- Malfunction Indicator Lamp (MIL) illuminated
- Possible reduced system performance or limp mode (depending on function)
- Failed emissions test or increased emissions
- Related subsystem not functioning (e.g., secondary air, purge, or other actuator function)
- Intermittent operation of the affected subsystem
What to check
- Retrieve all stored codes and freeze frame/Live Data with a scan tool
- Confirm code is current or pending; note conditions when it set
- Visual inspection of wiring harness, connectors, and mounting for damage or corrosion
- Check related fuse(s) and relay(s)
- Measure battery voltage at the connector with key ON
- Measure solenoid coil resistance (compare to spec) and check for continuity to PCM
Signal parameters
- Expected supply voltage: near battery voltage (~11–14.5 V) at solenoid supply pin with key ON or when commanded (varies by vehicle)
- Ground: low resistance to chassis (
- Coil resistance: typically low tens of ohms for solenoid coils (often 5–100 Ω) — consult vehicle spec
- PCM output: switched battery voltage or pulsed (PWM) ground depending on design
- Command duty cycle: 0–100% when controlled by PWM (scan tool can show duty %)
Diagnostic algorithm
- Read and record fault codes, freeze frame and live data. Note operating conditions when code set.
- Visually inspect solenoid, connector, and wiring for damage, corrosion, loose pins or poor grounding.
- Verify related fuses and relays. Replace if faulty.
- With key OFF, disconnect the solenoid connector. Measure coil resistance across the solenoid pins and compare to spec. An open or very low resistance indicates failure.
- Check supply voltage at the solenoid connector with key ON. Verify good battery voltage at the supply terminal.
- Check ground continuity from solenoid ground terminal to chassis ground. Repair any high-resistance ground.
- Command the solenoid ON with a scan tool (or safely apply battery voltage through an inline fuse) and confirm the solenoid actuates and the circuit draws expected current. If it does not, replace the solenoid.
- If solenoid tests good, backprobe PCM control terminal while commanding the solenoid. Verify PCM provides expected drive signal (voltage or PWM). If PCM driver is not switching, suspect ECM/PCM or wiring issue.
- Repair any wiring/connectors found faulty. Reassemble, clear codes, and perform drive cycle or test procedure to verify repair.
- If intermittent or still present after wiring/solenoid replacement, consult manufacturer technical service bulletins and consider ECM testing/replacement as a last resort.
Likely causes
- Damaged wiring or connector between PCM and solenoid
- Failed solenoid coil (most common component failure)
- Corroded connector or poor ground connection
- Faulty fuse/relay supplying circuit
Fault status
Status
Secondary switch solenoid circuit malfunction — electrical fault in solenoid/control circuit (open/short/poor connection or driver fault).
Repair difficulty: Medium
Diagnostic time: 0.5-2.0 hours
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