What do P0300 and P0191 together mean?

It means the computer recorded a random-misfire concern and a fuel-rail-pressure signal performance concern, but the pair alone does not identify the cause. In the Ford-specific systems covered by the references, P0300 is a general or random misfire code, while P0191 concerns fuel-rail-pressure sensor circuit range/performance or a rationality check. Those definitions are not universal service instructions for every vehicle. [S1] [S2]

P0191 does not, by itself, prove low fuel pressure, failed fuel delivery, or a defective sensor. The two codes may be related, or may require separate investigation. The right interpretation depends on the year, make, model, engine, applicable manufacturer information, and the conditions recorded when the codes set.

Write down the year, make, model, and engine, using the vehicle documents or identification information available to you. Record both exact codes and whether each is current, pending, or stored, if your scan-tool report shows that status. Keep the freeze-frame information and any other codes shown rather than relying on memory.

Also note when you noticed symptoms, such as rough running or a change under particular driving conditions, without trying to reproduce them. Save the report as-is and label anything you do not understand as “unknown.” A technician can decide which readings matter using information for your specific vehicle.

Give the technician observations and the scan report, not a diagnosis. For example, assuming the details in brackets are placeholders, you could say:

“Vehicle: [year] [make] [model], [engine]. Codes reported: P0300 [status] and P0191 [status]. Freeze-frame information available: [attach report or list values]. I noticed [symptom or ‘no noticeable symptom’] at [when it occurred]. Other codes: [list or ‘none shown’]. Please interpret these codes using the applicable service information and advise what checks are appropriate before replacing parts.”

This wording keeps a sensor fault, an actual pressure issue, and other possible explanations open. The cited Ford documents explain Ford-specific monitor definitions; they do not establish the cause on another vehicle or prescribe a repair. [S1] [S2]

Ask the technician to confirm the vehicle configuration and code status, review the freeze-frame context and any related codes, then consult the manufacturer service information for that exact engine and model year. Ask how they will distinguish a pressure-signal plausibility concern from a confirmed fuel-delivery problem, and what evidence would support any proposed part replacement. A code name alone is not that evidence.

Do not attempt fuel-system pressure tests or other hands-on diagnosis based on this guide. The sources define particular Ford monitors; they do not provide a safe procedure or establish which tests apply to your vehicle. Ask a qualified automotive professional to choose and perform any necessary checks. [S1] [S2]