P0455 means the EVAP monitor could not establish or hold expected sealing. Record filler condition and recent pump/tank service, prove purge and ESIM operation, then section the WK2 or WL system with regulated smoke. A missing cap, stuck-open valve or disconnected hose can look identical to the PCM. P0456 calibration bulletins do not automatically apply to this larger-leak code.
Match the symptom
Select the closest observation. The result changes the first test; it does not name a part to replace.
Record cap/filler state.
What can set P0455—and how to separate it
The order below is evidence-driven, not a universal probability ranking. Vehicle year, repair history, companion codes and the operating condition in freeze frame change where diagnosis should begin.
| Possible cause | Evidence that supports it | First useful test |
|---|---|---|
| Missing/loose/damaged cap or filler | Found condition/seal test | Filler inspection |
| Purge valve stuck open | Vacuum passes commanded closed | Sealing test |
| ESIM/vent or disconnected hose | Cannot seal/smoke exits | Function and smoke test |
| Canister, tank or pump seal leak | Sectional smoke | Low-pressure test |
| Leak-detection circuit fault | Implausible data/companion code | Circuit/function test |
Large leak can be a normal path held open
Purge and vent paths must seal during testing.
Verify mechanics, not only coil clicks.
Tank service leaves evidence
Pump seals and quick-connect lines deserve inspection. On a Grand Cherokee, also inspect above-tank connections after pump work and rear vapor-line routing after collision or underbody repair.
Smoke at safe pressure.
Platform controls component names
WK2 and WL packaging changed.
Use VIN-correct routing.
Save these scan-data clues
Freeze frame is a snapshot of conditions when the PCM matured the fault. It can distinguish an idle-only problem from a load, cold-start or monitor-completion problem. Clearing first throws that context away.
- Exact VIN, platform, model year and engine sales code
- All confirmed, pending and permanent companion codes
- RPM, load, coolant temperature and vehicle speed
- Relevant commanded and actual scan data
- Recent service, repairs and symptom conditions
- Manufacturer bulletin and current software/VIN status
A practical diagnostic workflow
- 01Identify platform
Routing differs.
- 02Preserve evidence
Record filler before changing.
- 03Test purge and ESIM/vent
Clicking is not sealing.
- 04Smoke safely
Section tank, canister and filler.
- 05Validate detection data
Circuit faults can imitate leakage.
- 06Complete monitor
Meet fuel/soak conditions.
Do not replace the most commonly named component from P0455 alone. Confirm the exact engine, preserve evidence and complete the manufacturer test sequence before ordering parts.
Why the model year changes the test
Grand Cherokee spans ZJ/WJ/WK/WK2/WL platforms and inline-six, V6, HEMI, diesel, turbo and 4xe powertrains. Oil-pressure control, EVAP hardware, cylinder banks, catalysts and cam sensors vary. Confirm platform, VIN, engine sales code, market and PCM calibration before using a bulletin or component diagram.
- WK2 and WL filler/ESIM routing differs.
- Diesel/4xe vapor systems require exact service information.
- Rear-collision and tank-service history is especially relevant.
- P0455 is not automatically covered by a P0456 PCM update.
P0455 FAQ
Is it always the cap?+
No.
Can I drive?+
Usually briefly without odor or wet fuel.
Can purge cause hard start?+
Yes.
Can tank service cause it?+
Yes, through seals or disconnected lines.
Can I use shop air?+
Not unregulated.
Why did a new cap fail?+
The fault may be elsewhere.
Does a P0456 flash apply?+
Not automatically to P0455.
What confirms repair?+
Passing leak test and monitor.
Ford diagnostic references
These documents explain Ford monitor operation. Exact pinpoint tests, connector views and specifications must be selected for the truck's VIN and model year in current service information.
01Mopar vehicle informationOfficial owner manuals and model-year engine, oil, warning and operating information↗02Stellantis/Mopar service informationFactory VIN-, platform-, engine-sales-code and calibration-specific service information↗