# Trouble code PO300



## 04claymax (Jun 7, 2017)

I recently bought an 04 Maxima 3.5 6 speed it had been sitting for some time and was told it needed coils. I changed the plugs,coils,injectors, replaced the plenum gasket and replaced some vacum lines now its throwing a PO300 code and runs rough any other suggestions greatly appreciated.


----------



## rogoman (Dec 16, 2004)

What was wrong with the car prior to you changing the plugs, coil pacs and injectors? A P0300 fault code means that there are multiple cylinders that may be misfiring. If the car didn't have a fault code prior to the parts replacement, then there may be a defective replacement component or the reassembly was incorrectly done. Here are some possible causes: 

- Improper spark plugs
- The injector circuit is open or shorted
- One or more defective Injectors
- Intake system vacuum leak
- Incorrect PCV hose connection

Since you replaced the plenum gasket, I would strongly suspect a vacuum leak there. Use a vacuum gauge for good accuracy; most auto parts stores sell vacuum gauges. To check the intake system for a vacuum leak, attach a vacuum gauge to a full vacuum source, usually somewhere on the intake manifold. With the engine fully warmed up, the reading at idle should be 18 - 20 InHg. At 3,000 RPM, it should be around 21 InHg.

If readings are under 18 InHg, check the intake manifold nuts to make sure they are tight. The gasket may have failed; spray a water mist at the gasket to see if the gauge reading changes. Also check the intake plenum bellows at the throttle body and at the MAF for cracks or loose clamps.


----------

