Can the various engine sensors be tested. Our '07 Fit won't start. On the upside, a new battery and starter got my '91 Civic wagon running this week so all is not gloomy.

 

They can be tested, at least in theory. Exactly how to do so, and what specialized equipment might be needed, and whether you'd have to remove the part from the engine, all vary with different sensors. Not all the sensors are important for the engine being able to start, either.

What exactly happens when you go to start it? If the engine doesn't even crank, it's not a sensor issue; it's a starter motor or associated circuitry problem.

If it does crank, what does the little key light on the instrument panel do? If it's staying on or flashing repeatedly, the immobilizer is preventing the engine from starting. It should just flash on briefly. (I think the '07s have an immobilizer system; but if I'm wrong, please ignore this paragraph.)

 

In some cases it can be just a bad ground, or a loose battery terminal, keeping the car from starting or even cranking. Start simple.