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P0499 – Evaporative Emission System Vent Control Circuit High (OBD2)

P0499 means "Evaporative Emission System Vent Control Circuit High". The voltage in the evaporative emission (EVAP) system circuit is above the permitted maximum. For most sensors this points to an open signal or ground wire – the signal "floats" up to the reference voltage – or to a short to voltage.

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P0499 quick summary

Fault codeP0499
MeaningEvaporative Emission System Vent Control Circuit High
SystemPowertrain (Engine & Transmission)
VehiclesGeneric OBD2
SeverityLow
Can I drive?Yes. EVAP faults rarely affect driving, but fuel vapour escapes and the car may fail an emissions test. A strong fuel smell should be checked promptly.
Repair cost€0 – €400

How the EVAP system works

The evaporative emission system stores fuel vapour from the tank in a charcoal canister and burns it in the engine, so it does not escape into the air; these codes cover leaks, purge flow and the EVAP valves.

Fuel vapour from the tank is collected in an activated charcoal canister. When the engine is warm, the purge valve opens and the engine draws the vapour in and burns it. A vent valve lets fresh air into the canister; on many cars it can close so the system can be leak-tested.

The control unit regularly tests the system: it seals it, applies vacuum (or pressure with a leak detection pump) and watches whether it holds. Small and large leak codes (P0442, P0455, P0456) mean the system did not hold. Other codes cover the purge valve, vent valve, pressure sensor and purge flow.

By far the most common cause of a leak code is a fuel cap that is loose, missing or has a damaged seal. Other causes are cracked hoses, a leaking canister or valves that stick. Most EVAP faults do not affect how the car drives, but a purge valve stuck open can cause rough idle and hard starting after refuelling.

On VAG vehicles (VW, Audi, Škoda, Seat, Cupra), VCDS and older VAG testers can show this code as the five-digit VAG code 16883 (P0499). Both numbers mean the same fault.

Symptoms of P0499

  • Check Engine light on
  • Fuel smell around the car
  • Rough idle or hard starting after refuelling (purge valve stuck open)
  • Fuel pump nozzle clicks off repeatedly when refuelling (vent problem)
  • Usually no driving symptoms

Most common causes of P0499 (most likely first)

Mechanic's Pro Tip

Fix the cheap things first: a new fuel cap seal costs very little and solves a large share of small-leak codes. And do not keep filling after the nozzle clicks off – overfilling pushes liquid fuel into the charcoal canister and ruins it.

  1. 1.Open circuit in the signal or ground wire

    Most likely

    A loose ground connection is a very common cause.

  2. 2.Signal wire shorted to the 5 V reference or battery voltage

  3. 3.Faulty evaporative emission (EVAP) system

  4. 4.Loose, missing or damaged fuel cap

    The most common cause of leak codes.

  5. 5.Cracked EVAP hoses or connections

    Plastic lines become brittle with age.

  6. 6.Leaking or saturated charcoal canister

    Overfilling the tank can push fuel into the canister.

Step-by-step diagnostic guide

  1. 1

    Check the fuel cap

    Check that the fuel cap is present, tight and the seal is not cracked. Clear the codes and see if they return.

  2. 2

    Check signal and ground

    With the ignition on and the evaporative emission (EVAP) system plugged in, back-probe the signal and ground wires. A signal stuck at reference or battery voltage points to an open ground/signal wire or a short to voltage.

    Multimeter: Ground wire to battery negative: below about 0.1 V. Signal stuck at about 5 V (or 12 V) = open or short to voltage.

  3. 3

    Read codes and run the EVAP test

    Read all codes and, where the scan tool supports it, run the EVAP system test.

  4. 4

    Smoke-test the system

    A smoke test through the service port or vent side finds small leaks in hoses, the canister and the filler neck.

  5. 5

    Test the purge and vent valves

    Command the valves with the scan tool and check that they open and close. Check their resistance and wiring.

    Multimeter: Typical purge valve resistance: about 20–35 Ω. With the engine idling and the purge valve not commanded, there should be no vacuum at the canister side.

  6. 6

    Repair and verify

    Replace the faulty part and drive the car through a full EVAP test cycle (often requires a cold start and a certain fuel level) to confirm the repair.

Frequently asked questions

Can I keep driving with P0499?

Yes. EVAP faults rarely affect driving, but fuel vapour escapes and the car may fail an emissions test. A strong fuel smell should be checked promptly.

How much does it cost to fix P0499?

Typical repair costs are 0 € – 400 €, depending on the cause and the vehicle. The most common fixes are: open circuit in the signal or ground wire; signal wire shorted to the 5 V reference or battery voltage; faulty evaporative emission (EVAP) system. Always confirm the cause with the tests above before buying parts.

Can a loose fuel cap really turn on the Check Engine light?

Yes. The control unit tests the EVAP system for leaks, and an unsealed fuel cap is the most common reason for a leak code.

Why does the code take days to clear after the repair?

The EVAP test only runs under specific conditions (for example a cold start and a certain fuel level). The code clears after the test passes, which may take several drive cycles.

What is the difference between P0499 and similar codes?

P0499 belongs to a group of codes for the same component that differ in the type or location of the fault: P0496 – Evaporative Emission System High Purge Flow; P0497 – Evaporative Emission System Low Purge Flow; P0498 – Evaporative Emission System Vent Control Circuit Low; P2024 – Evaporative Emission System Fuel Vapor Temperature Sensor Circuit; P2025 – Evaporative Emission System Fuel Vapor Temperature Sensor Performance; P2026 – Evaporative Emission System Fuel Vapor Temperature Sensor Circuit Low. The tests on this page apply to the whole group; the type of fault tells you where to start.

Related fault codes

This guide is for information only. Working on vehicles can be dangerous — if you are unsure, have the car checked by a qualified mechanic. Values can differ between model years; always check the manufacturer's repair information.