What do P0300 and P0420 together mean?

They record two different observations. Ford defines P0300 as “Random Misfire Detected.” [S2] In the cited Ford system, P0420 means “Catalyst System Efficiency Below Threshold, Bank 1 (or Y-pipe).” [S1] The definitions are specific to the cited manufacturer documents; verify how they apply to your vehicle with its year, make, model, and engine.

The pair does not prove the catalyst caused the misfire, nor does it by itself establish that the catalyst has failed. Treat them as clues for a vehicle-specific diagnostic check. Keep the codes separate in your notes, and avoid replacing parts based on the code pair alone.

No—not from these codes alone. The Ford OBD summary describes monitor prerequisites and says that, on the documented system, certain other faults, including misfire-related faults, may disable the catalyst monitor for that key cycle. That logic is Ford- and system-specific, not a universal rule. [S1] It supports paying attention to chronology and code status rather than assuming one code caused the other.

For example, suppose a scan report lists P0300 as pending and P0420 as stored. That is only an illustrative record, not a diagnosis; it does not establish when either condition began or what caused it. Ask the technician to review the actual scan information and applicable service data for your vehicle.

Write down the vehicle’s year, make, model, and engine, plus the exact scan-tool wording if available. For each code, record whether the report calls it pending, stored, or another status; keep the report’s exact wording rather than trying to interpret it yourself. Save any available freeze-frame information and the date or circumstances when the codes appeared. Note whether both were present in the same scan or appeared at different times.

If you do not have a detail, mark it “unknown” rather than guessing. A scan report may not provide every useful detail, and the supplied code definitions do not explain how to interpret every status or freeze-frame field for every vehicle. The vehicle-specific service information and scan data are needed to interpret the combination. [S2]

Use a neutral handoff that reports observations without suggesting a failed part. For example, under the explicit assumption that the information below is available, you could write:

“Vehicle: [year] [make] [model], engine [engine]. Scan on [date]: P0300 — status [as reported]; P0420 — status [as reported]. They appeared [together / at different times / unknown]. Freeze-frame information: [attach or summarize / unavailable]. Please assess both records using service information for this vehicle and explain what the available scan data does and does not establish.”

Ask whether the codes were current in the same scan, what vehicle-specific data or checks support the next diagnostic step, and whether there are limitations in the report. Do not present the example as a diagnosis or replace the placeholders with guesses.