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P1491

Vacuum Cut Valve Bypass Valve.

  • Nissan
  • Powertrain
  • Manufacturer code
  • Low 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

On Nissan models P1491 relates to the vacuum cut valve bypass valve, a solenoid-operated valve in the evaporative emission (EVAP) system near the fuel tank vapour line. The engine control unit commands this valve and monitors its circuit and operation. The code sets when the circuit voltage is not what it expects, or when the valve does not behave as commanded during the EVAP system checks. Exact valve location and test conditions vary by model and engine.

Symptoms

  • Malfunction indicator lamp on
  • Smell of fuel vapour around the car or fuel filler
  • Difficulty refuelling, such as the pump clicking off repeatedly
  • Failed emissions or inspection readiness test
  • Usually no noticeable change in driveability

Possible causes

  • Faulty vacuum cut valve bypass valve solenoid (open or shorted coil, or stuck valve)likely
  • Damaged, corroded or loose connector at the bypass valvelikely
  • Open circuit, short to earth or short to voltage in the valve wiring or its power supplypossible
  • Blocked, split or disconnected EVAP vapour or vacuum hoses affecting the valve operationpossible
  • Faulty vacuum cut valve or related EVAP component upsetting the system responsepossible
  • Fault in the engine control unit driver circuitrare

How to diagnose

  1. Read all stored and pending codes and note the freeze frame data, then check for other EVAP codes that point to the same area
  2. Check the fuel cap is fitted correctly, in good condition and the correct type for the car
  3. Visually inspect the bypass valve, its connector, the wiring and the nearby EVAP hoses for damage, corrosion, chafing or poor routing
  4. Use a scan tool's active test, if the model supports it, to command the bypass valve and listen or feel for it operating
  5. Measure the solenoid coil resistance and compare with the manufacturer's specification for that model
  6. Check the valve's power supply and control wire for continuity, shorts and correct voltage using the wiring diagram
  7. Inspect the vapour and vacuum hoses and the valve for blockage, cracks or loose joints
  8. Clear the code and road test to confirm it does not return, running the EVAP monitor if needed

How to fix

  • Replace the bypass valve if its coil resistance is out of specification or it fails the active test
  • Clean, repair or replace the damaged connector terminals
  • Repair open or shorted wiring between the valve and the control unit, or restore the power supply
  • Replace cracked, blocked or disconnected EVAP hoses and fit the correct cap if it is faulty
  • Replace the vacuum cut valve or other faulty EVAP part if testing shows it is at fault

Can I keep driving?

Low severity

The car can normally be driven as the fault affects the evaporative emission system rather than engine running. Book it in soon, especially if you smell fuel, as a vapour leak is a fire and environmental concern.

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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