What do P0442 and P0455 together mean?

They are both EVAP-system leak-monitor records, but they represent different leak criteria in the cited Ford OBD-II document: P0442 is associated with a 0.040-inch leak test, while P0455 is associated with a gross-leak monitor threshold. That makes the pair a clue that different monitor tests recorded a problem—not a map showing where the problem is, and not proof of two separately located leaks. [S1]

Those details are Ford-specific. Other vehicles may use different thresholds or test conditions, so don’t treat the Ford description as the repair specification for your car. The pair alone cannot identify a failed cap, hose, valve, tank, or other part. A qualified technician should interpret the codes using information for your exact vehicle.

Not by itself. Two stored codes do not establish which monitor ran first, whether the same condition triggered both, or whether a leak changed size. The Ford document notes that evaluation timing and thresholds can vary with fuel fill and ambient temperature, so the surrounding test context matters. [S1]

Ask for the status and any available freeze-frame or other scan-tool records for each code. Those records may provide context for a technician’s assessment, but the code pair alone does not establish a complete chronology or explain the cause. If the scan tool does not show relevant context, note that it is unavailable rather than guessing.

Write down the year, make, model, and engine, along with the exact codes and their status as shown by the scan tool. Copy any available freeze-frame or other scan-tool context as displayed, without assuming it identifies the failed part.

For example, you could hand over this copyable note:

“Vehicle: [year] [make] [model], [engine]. Codes: P0442 [status shown]; P0455 [status shown]. Freeze-frame/context: [copy displayed values, or ‘not available’]. Symptoms or relevant observations: [brief description, or ‘none noticed’]. Please check both records against the applicable service information and explain what further diagnosis is needed before recommending a part.”

Keep the codes as scanned; don’t replace a component based only on the pair.

Ask them to start with the vehicle’s applicable service information and the records for both codes, then explain how the code statuses and any available context affect their next diagnostic step. You can ask whether the two codes were recorded in the same event or at different times if the available records can show that, and what evidence would be needed to narrow down the cause.

For example: “Could you confirm how P0442 and P0455 apply to my [year/make/model/engine], review their status and available context, and tell me what test or observation supports any proposed repair?” That keeps the discussion focused on evidence rather than assuming a particular leak location. The cited Ford document explains monitor criteria and test context; it does not establish the correct diagnosis or repair for your specific vehicle. [S1]