Failed leak detection pump assembly, where the internal switch or diaphragm no longer responds to the computer's command.; Damaged or corroded wiring and connector pins at the pump, especially at the rear of the vehicle where road splash attacks the harness.; Blocked or disconnected evaporative emissions hose between the pump and the fuel tank, preventing the pump from building or releasing pressure.; Stuck or contaminated leak detection pump filter, which restricts airflow and prevents the switch from seeing the expected pressure change.; Faulty fuel tank pressure sensor sending an implausible signal that makes the computer misjudge the pump's performance.; PCM software or calibration issue on certain GM trucks and SUVs that misinterprets a normal pump switch signal.

DTC codes / GM codes / P1494

P1494 · GMLeak Detection Pump Switch Or Mechanical Fault

Q: What does code P1494 mean?

Leak Detection Pump Switch Or Mechanical Fault. P1494 on a GM vehicle means the engine computer has commanded the leak detection pump to run its evaporative emissions self-test, but the pump's internal switch did not report the expected change of state, or the pump failed to move air as commanded. The computer is not seeing the switch transition it expects during the pump's on/off cycle, so it flags the circuit or the pump mechanism as faulty.

P1494 on a GM vehicle means the engine computer has commanded the leak detection pump to run its evaporative emissions self-test, but the pump's internal switch did not report the expected change of state, or the pump failed to move air as commanded. The computer is not seeing the switch transition it expects during the pump's on/off cycle, so it flags the circuit or the pump mechanism as faulty.

SystemPowertrain

Code typeGM specific

SeverityMedium

Fix difficultyIntermediate

AreaAuxiliary emission controls

Symptoms you may notice

Illuminated check engine light, often with no other noticeable symptoms.
A slight fuel odor around the rear of the vehicle if the pump or its plumbing is leaking.
Failed state emissions inspection because the evaporative system monitor will not complete.
Rough idle or a stumble after refueling in some cases where the pump stuck open creates a small vacuum leak.
Intermittent check engine light that clears and resets over several drive cycles as the pump occasionally works.

Common causes, most likely first

Failed leak detection pump assembly, where the internal switch or diaphragm no longer responds to the computer's command.
Damaged or corroded wiring and connector pins at the pump, especially at the rear of the vehicle where road splash attacks the harness.
Blocked or disconnected evaporative emissions hose between the pump and the fuel tank, preventing the pump from building or releasing pressure.
Stuck or contaminated leak detection pump filter, which restricts airflow and prevents the switch from seeing the expected pressure change.
Faulty fuel tank pressure sensor sending an implausible signal that makes the computer misjudge the pump's performance.
PCM software or calibration issue on certain GM trucks and SUVs that misinterprets a normal pump switch signal.

How to diagnose and fix it

Connect a scan tool and command the leak detection pump on and off while watching the switch status parameter to confirm whether the switch actually changes state.
Visually inspect the pump, its connector, and the evaporative hoses at the rear of the vehicle for corrosion, cracks, or disconnected lines.
Check for battery voltage and a good ground at the pump connector with a multimeter while the pump is commanded on.
Remove the pump and bench-test it with 12 volts to confirm the motor runs and the switch contacts open and close as the pump cycles.
Inspect and replace the pump filter if it is clogged with dust or debris, then retest the system.
If the pump and wiring test good, compare fuel tank pressure sensor readings against a known-good value and replace the sensor if it is skewed.
Clear the code and run the evaporative system monitor to completion to confirm the repair before returning the vehicle.

What the pump switch test really checks

The leak detection pump switch is a simple reed or mechanical contact inside the pump, and a pump that runs quietly can still fail this code because the switch never closes.
A common misdiagnosis is replacing the fuel cap or purge valve first, since those cause other evaporative codes, but P1494 specifically points at the pump's own switch or mechanism.
Corroded pins at the pump connector are extremely common on older GM trucks, and a wiggle test of the harness while watching the switch parameter often reveals the fault.
A restricted pump filter can mimic a failed pump because the motor strains and the switch timing falls outside the expected window, so always inspect the filter before condemning the pump.
On some GM platforms the evaporative system monitor will not run unless the fuel level is between roughly one-quarter and three-quarters full, so a low tank can prevent verification after repair.

Can I keep driving with P1494?

The vehicle remains safe to drive because this fault affects only the evaporative emissions system and does not change engine performance, but you should repair it before an emissions inspection. Avoid topping off the fuel tank past the first click, since a full tank can push liquid fuel into the pump and worsen the fault.

Tools that help

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Guidance is general and written for GM vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.