What do P0455 and P0456 together mean?

Not necessarily. The two stored codes do not prove that two leaks exist at the same time, and neither code pair alone identifies a failed part or leak location. In the Ford system covered by the cited document, P0455 relates to a gross-leak monitor, while P0456 relates to a 0.020-inch leak test. Those are separate monitor evaluations; the document also says that the small-leak test's timing or threshold can vary with fuel fill and ambient temperature. These details are specific to the Ford system described, not universal definitions or test rules for every vehicle. [S1]

Treat the pair as a reason to check which tests ran and when, rather than as a parts-replacement instruction. A qualified technician can check the service information for your exact vehicle and interpret the stored data.

Record the year, make, model, and engine if known. Note whether each code is current, pending, or stored/history according to your scan tool; use the tool's exact wording if unsure. If available, save the freeze-frame details associated with each code rather than clearing anything first. Also note when the warning light appeared, whether either code returned after a prior repair, and any recent refueling or other relevant event you remember. These are observations to help frame the diagnostic check, not proof of a cause.

A useful handoff could be: “Vehicle: [year, make, model, engine]. Codes shown: P0455 [status] and P0456 [status]. Freeze-frame/context: [available details]. Warning-light timing or recent relevant events: [brief notes]. Could you check the applicable service information and tell me what evidence supports the next test?”

Not by itself. A scan-tool list may show codes together without establishing the order in which their monitor evaluations occurred. The Ford document notes that the small-leak monitor's timing and threshold can vary with fuel fill and ambient temperature, so a paired record does not establish simultaneous leaks or a sequence. [S1]

Ask the shop whether the available status and freeze-frame information shows when each code was recorded and whether either is pending or stored. If the tool does not retain that context, say so; do not infer a timeline from the display order. The cited document does not establish the test strategy for your particular vehicle, the actual leak location, or which component—if any—has failed.

Ask for a diagnosis based on the exact vehicle's applicable service information, not an assumption that the code pair automatically calls for a particular part. For example:

1. “Does the manufacturer information for my year, model, and engine define these codes or monitor steps differently?”
2. “Which code statuses and freeze-frame details are available, and do they clarify when the codes were set?”
3. “What test would distinguish a stored monitor result from a currently reproducible issue?”
4. “What evidence supports the proposed cause and repair, and how will you verify the result?”

You can share the code list and observations yourself, but leave testing and repairs to a qualified technician. The source describes one Ford system's monitor context; it does not diagnose your vehicle or establish a universal procedure. [S1]