Mazda 6 Service Manual: No.5 engine stalls-after start/at idle

No.4 hard to start/long crank/erratic start/erratic crank

No.6 cranks normally but will not start

Other materials:

Mazda 6 Service Manual: Intermittent concern troubleshooting
  Vibration Method 1. If malfunction occurs or becomes worse while driving on a rough road or when engine is vibrating, perform the steps below. Note There are several reasons vehicle or engine vibration could cause an electrical malfunction. Some of the things to check for are: ...

Mazda 6 Service Manual: Precautions before welding
Vehicles have various electrical parts. To protect the parts from excessive current generated when welding, be sure to perform the following procedure. 1. Turn the ignition switch to the LOCK position. 2. Disconnect the battery cables. 3. Securely connect the welding machine to the ground nea ...