Nissan Sentra 2007 P0300 and P0420 Codes: Causes, Fixes, and Cost Estimates
Hello everyone! I'm new to this forum and only know how to change oil when it comes to car maintenance. Recently, I bought an OBDII scanner and discovered two persistent trouble codes on my 2007 Nissan Sentra: P0300 (Random/Multiple Cylinder Misfire) and P0420 (Catalyst System Efficiency Below Threshold). These codes are causing my engine to idle unevenly, stumble during driving, and trigger the check engine light—especially noticeable with a manual transmission. I'm considering taking my car to a mechanic, but I'd like to understand what these codes mean and get an estimate of repair costs before spending money. Does anyone have experience diagnosing or fixing P0300 and P0420 on a 2007 Nissan Sentra? What are the most common causes, and how much might a professional fix cost? Any insights or advice would be greatly appreciated—thank you in advance for your help!
P0300 can often lead to P0420 because a misfiring engine produces harmful emissions that reduce catalyst efficiency. The P0300 code indicates poor engine performance with multiple cylinders involved—possible causes include faulty spark plugs, ignition coils, fuel injectors, or vacuum leaks. To get an accurate diagnosis, it's recommended to share a datalog and freeze frame data from the OBDII scanner. This helps technicians pinpoint the exact issue without guesswork.