What are the four basic steps of troubleshooting a modem?

These are the four basic steps to troubleshooting:
Step 1 - Analyze the circuit
First, see if the circuit operates correctly. If the circuit operates correctly, there is no need to troubleshoot and this would be the last step. If the circuit does NOT operate correctly, then you must go to the next step.

Step 2 - Check the setup
If the circuit is not operating correctly, check to see if you have overlooked something simple. Check connections and buttons to see if they are set correctly. If you checked the setup and the setup is good, but the the circuit does not operate correctly, go on to step 3.

Step 3 - Calculate and measure values
First, calculate to determine the values that should be present in the circuit that is not working properly by using I = V/R. Make voltage, current or resistance measurements. In most cases, the voltage measurements are made first. Voltage measurements provide useful information about the circuit and are made without breaking the circuit. If more information is needed, make current or resistance measurements. Remember, the circuit MUST be broken to make these measurements. Make as many measurements as needed. As the pieces of the puzzle begin to fall into place, identifying the fault becomes easier.

Step 4 - Check the results
Compare the calculated to measured values. You should have enough information to identify the fault. Replace the faulted component and perform step 1 again. If the circuit operates properly, the circuit is fixed.