Need help clearing p0303

Do you have a “hybrid aware” scan tool to pull codes, or Techstream?

Is the engine in “Maintenance Mode” when the misfire starts?

If you have a spare available, change the upstream O2 sensor. It’s the one controlling the A/F ratio. If the signal is falling off, it could be leaning/enrichening the A/F ratio to the point of misfire.

 

The ECUs generally have modes which they use as default when a sensor is disconnected or when starting the car from cold. Think parameters and subroutines coded into the program that controls the air/fuel mixture. They could be reprogrammed with a specialty tool but that is beyond what most can do. Once running warm, the ECU runs a different subroutine that gradually learns from the values received from the Mass Airflow Sensor in the intake and the O2 sensors located in the exhaust combined with accelerator position, knock sensor and .....

I forget, have you swapped the #3 coil with another to see if the problem follows the coil? Examined the spark plug comparing it's color with others? Checked the anti-freeze level to see if it is low and could be leaking through a gasket into that cylinder?

 

How does it run at WOT (wide open throttle) ? The reason I ask is it is when a car runs the richest.
There are two types of misfires, one without fuel and one without spark. People that are good at reading scan data can tell which one it is.

 

It's not direct injection, the injectors protrude into the intake port, not the combustion chamber. So not sure how compression could be effected. Still, an improperly seated injector could cause a vacuum leak I think?

 

Can you read you O2 sensor output? When I mentioned Open Loop and Closed Loop, the O2 sensor is not used at all in Open Loop. Open loop is used at start up and at WOT (wide open throttle). Your scan tool should show you when it switches between the two. If your misfire is gone in open loop it tells you one of the input not being used in closed loop is bad. I don't know all the inputs to the ECU but the O2 sensor is a biggie.