Can a mobile mechanic fix a car that won't start?

The realistic yes list

A large share of no-starts are electrical or supply problems rather than mechanical ones, and those are exactly what roadside work is good at.

• Battery. Flat, failed, or corroded and loose terminals. Very common, and very often the whole story.
• Charging system. A failed alternator is sometimes replaceable on site, depending entirely on how buried it is in that particular engine bay. Battery or alternator?
• Starter motor. Accessible on some vehicles, deeply unpleasant on others. The diagnosis is quick either way.
• Belts. A snapped serpentine belt strands a perfectly healthy engine and is frequently a roadside fix.
• Fuses and relays. A dead fuel-pump relay presents exactly like a catastrophic failure and costs very little.
• Fuel. Out of it, or a gauge lying about it. Fuel delivery.
• Immobiliser and key faults. A car that cranks but will not run because the key is not being recognised is not a broken engine.
• Loose or corroded connections and bad grounds. Unglamorous, extremely common.

The realistic no list

Being straight about this is what makes the yes list believable.

• Anything needing the vehicle in the air. If it needs a lift, it needs a shop.
• Internal engine work. Timing belts or chains that have jumped or snapped, compression failures, head gasket work.
• Transmission internals. Not roadside work in any sense.
• Anything after severe overheating. An engine that was run hot needs proper assessment before anyone tries to start it again.
• Vehicle-specific parts nobody has tonight. The diagnosis can be done; the repair waits on the part.
• Anywhere genuinely unsafe. A live traffic lane or a blind curve gets the vehicle moved first. That is not upselling a tow, it is refusing to have someone killed over a starter motor.

Why the diagnosis is worth it even when the answer is "tow it"

This is the part people miss. Suppose the mechanic comes out and confirms the timing chain has gone — genuinely not a roadside repair. You have still gained:

• You know what is wrong before you decide where to send it.
• You can judge whether the vehicle is worth the repair at all before paying to move it.
• You are not paying a shop a diagnostic fee to reach the same conclusion.
• You can choose the destination on the basis of the actual fault rather than proximity.

The alternative sequence — tow first, diagnose second — means you have already spent the tow before you knew whether the car deserved it.

How it usually goes on the phone

You describe the symptoms. Nothing at all when you turn the key, versus a single click, versus cranking without catching, versus dying while driving — those point in genuinely different directions, and a good dispatcher will steer the call from what you say.

Sometimes the answer at that point is already "that is almost certainly a tow" — an engine that was overheating, say. Sometimes it is "that sounds like a battery, we will come and test it". Most often it is "we will not know until we look, and looking is the cheap part".

What nobody can do

Tell you what is wrong with your car from a photograph or a description. A photo genuinely helps — it shows a dispatcher the warning light, the damage, where the car sits and what equipment the job needs. It is not a diagnosis and should never be sold as one. Anyone offering you a firm fault from a picture is guessing, and eventually they will guess wrong in a way that costs you an engine.