A sensing method and system to protect printer print heads from substrate media contact comprising a trip wire sensor to detect and signal a printer control system that substrate media carried by transport media is positioned to strike or contact the print head. The trip wire sensor is located upstream of the print heads, a controlled distance above the transport media in the direction normal to the plane of the media, and comprises a trip wire operatively connected to at least one transducer. Substrate media exceeding the height requirements associated with the print heads contact the trip wire sensor causing the generation of an electrical signal which is received by the printer control system which then takes corrective action to prevent damage to the print heads, for example, by raising the print heads further away from the transport media and allowing the non-conforming substrate media to pass through and be purged. Also disclosed is a protective guide positioned to constrain substrate media from going over the top of the trip wire which can result in the substrate media becoming jammed in the trip wire sensor.