I just bought a 2004 Mazda 6 2.3 and to start it you had to have foot on gas and let it idol rough for a couple minutes until it started idoling normal. The check engine light said cam shaft sensor and upstream o2 sensor. I replaced cam shaft sensor and crank shaft sensor and now the car won’t start at all. It cranks and sometimes backfires. Could the o2 sensor cause it not to start? If not what else could it be. Fuel pump turns on.