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P1491

Secondary Switch Solenoid Circuit Malfunction.

  • Other
  • Powertrain
  • Manufacturer code
  • Medium severity

How this code is built

P
SystemPowertrain
1
Defined byThe manufacturer
4
AreaAuxiliary emission controls
91
FaultThe specific fault in that area

What it means

P1491 is listed as 'Secondary Switch Solenoid Circuit Malfunction'. The control unit has detected an electrical fault in the circuit that drives a secondary switching solenoid, such as an open circuit, a short, or a reading that does not match the commanded state. Which system that solenoid controls varies by manufacturer and model, so confirm the exact definition for the vehicle in the manufacturer's data.

Symptoms

  • Engine warning light on
  • Slightly rough running or hesitation, depending on what the solenoid controls
  • Reduced performance or a change in how the engine pulls at certain revs
  • Poor fuel economy
  • No obvious symptoms other than a stored fault

Possible causes

  • Damaged, chafed or corroded wiring to the solenoid (open circuit, short to earth or short to supply)likely
  • Loose, corroded or water-damaged connector at the solenoid or control unitlikely
  • Failed solenoid with an internal open or shorted coillikely
  • Blown fuse or faulty relay supplying the solenoid circuitpossible
  • Low battery voltage or a poor earth connection affecting the circuitpossible
  • Fault in the control unit output driverrare

How to diagnose

  1. Read all stored and pending codes and note the freeze frame data, then check for related codes that point to the same system.
  2. Look up the exact definition and solenoid location for the model and engine in manufacturer data, as it varies between vehicles.
  3. Visually inspect the solenoid, its vacuum or hydraulic connections where fitted, the wiring and the connector for damage, heat, rubbing, corrosion or moisture.
  4. Check the relevant fuse and relay, and check battery voltage and the main earth connections.
  5. With the ignition on, check for supply voltage and a good earth at the solenoid connector, following the wiring diagram.
  6. Measure the solenoid coil resistance and compare it with the specification for the vehicle.
  7. Command the solenoid with a bidirectional scan tool, if supported, and listen or feel for operation while watching the live data.
  8. Check wiring continuity and insulation between the solenoid and the control unit, and only suspect the control unit once everything else is proven good.

How to fix

  • Repair or replace damaged wiring and fix any chafing point that caused it.
  • Clean, reseat or replace corroded or loose connectors and terminals.
  • Replace the solenoid if its resistance or operation is out of specification.
  • Replace the faulty fuse or relay after finding why it failed.
  • Restore battery condition or repair poor earth connections.
  • If the control unit is confirmed faulty, repair or replace it and have it programmed as the manufacturer specifies.

Can I keep driving?

Medium severity

The car can usually be driven short distances if it runs normally, but get it checked soon because the effects depend on what the solenoid controls. Avoid driving if there is reduced power, poor running or other warning lights.

Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.

This code for other makes

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