Emissions code
P0456: Very small fuel vapor leak
After the car is shut off, the computer seals the fuel-vapor system and checks whether pressure holds. This code means it leaks down very slowly, about as much as a hole the size of a pin. Because the leak is tiny, it is the slowest of the vapor codes to track down.
- Safe to drive?
- Usually yes, for now
- Most common cause
- Aging gas cap seal
Safe to drive; it does not affect how the car runs, but the light will stay on and it will fail an emissions check.
How this range is built
| Part of the job | Low | High |
|---|---|---|
| Parts | $8 | $40 |
| Labor: 0 to 0.3 hours × $125 to $175 an hour | $0 | $53 |
| Fees: shop supplies, disposal, diagnostics | $0 | $10 |
| Total, rounded | $5 | $110 |
The hourly rate is the BLS Occupational Employment and Wage Statistics, May 2025 average pay for this trade in Boston, $31.54 an hour, times 4 to 5.5 to cover what a shop bills (overhead, tools, insurance, profit). Parts are a band from budget aftermarket to the manufacturer part. Full method.
Not a repair shop, estimates only. Typical range, estimate. Not a quote. Your car, parts and shop change the number.
This is the range for the most common fix, gas cap replacement. Which repair your car needs depends on a proper diagnosis.
What you may notice
- Check-engine light on
- No change in how the car drives
- Light may come and go between trips
Common causes, most likely first
- Aging gas cap seal
- Pinhole in a vapor hose or fitting
- Leaky purge or vent valve
- Small crack in the filler neck
- Charcoal canister seam
Check this first
Replace the gas cap if it is more than a few years old, then wait a few days of driving. Tell the shop you already tried it so they go straight to a smoke test.
Other repairs that sometimes fix P0456
| Repair | Typical range, Boston |
|---|---|
| EVAP vent valve replacement | $120 to $360 |
| EVAP purge valve replacement | $85 to $260 |
| Diagnostic scan (check engine light) | $60 to $290 |
Before you pay
- Ask the shop what test showed this part is the cause, not just that the code appeared.
- Ask for parts and labor listed separately, and for the hours.
- Ask for a written OK before any extra work. Massachusetts rules require written authorization before repairs.
Got a quote already? Check it against the range. Want to see if your car has a recall tied to this problem? Check recalls by VIN.
Need to read a code yourself? OBD-II scanners on eBay (affiliate link, disclosure).
P0456 questions
Why is this so hard to fix?
The opening is so small that a quick look finds nothing, so shops use a smoke machine and trace the vapor lines one by one. That time is why this code can cost more to diagnose than it first seems.
Will it hurt gas mileage?
Almost never. A pinhole leak lets a few fumes out but does not change how the engine runs or burns fuel. The only real effect is the lit check-engine light and a possible inspection failure.
Can I ignore it until inspection?
You can keep driving, but most inspection programs will not pass a car with the light on. Plan to get it fixed before your sticker is due, and keep any receipts to show the repair was made.
How much does it cost to fix P0456?
The most common fix, gas cap replacement, has a typical range of $5 to $110 in Boston. It is an estimate built from parts prices and local labor wages, not a quote. Your car and shop change it.
P0456 repair cost by city
Related codes
- P0455: Large fuel vapor leak
- P0442: Small fuel vapor leak
- P0457: Fuel vapor leak, gas cap may be loose
- P0446: Fuel vapor vent circuit problem
More emissions codes
Code meaning written in our own words. Some codes mean different things on different makes, so check your scan tool or manual. Not a repair shop, estimates only.