What makes the photo useful?
Ask for one image that shows where the close-up was taken and another that clearly shows the suspected damage. The explanation should identify the furnace model, which exchanger section is pictured, and what the technician believes the mark is. A dark line in a cropped image, without location or explanation, leaves room for uncertainty.
The important question is not how many commenters agree. It is whether the original evaluator can connect the image to an observed defect and the model's inspection requirements. Ask whether the photo is the finding itself or supporting evidence for a finding established by other checks.
We recommend separating two decisions: confirming the condition, then choosing the supported repair or replacement. If the explanation remains unclear, arrange a qualified second assessment and share the original images and report. Request a comparison of the same suspected location. That is more useful than two unrelated general opinions.
What a second opinion should answer
Does it confirm the first finding, identify a different condition, or leave the area unresolved? What method was used? What instructions apply to operating the furnace now? Which work would address the confirmed condition, and what would remain unchanged?
Do not remove parts, light the burners with the cabinet open, or carry out a home leak test to make the photo more convincing. Internal inspection belongs with the qualified evaluator and applicable manufacturer instructions.
The next action is to request the location, finding, and use restriction in writing. A photo should help you understand the recommendation. It should not leave you deciding furnace safety from a phone screen.