When your 2018 Nissan Altima displays the P0305 code, the engine control module has detected repeated misfires in cylinder 5. The 2018 Altima (final year of the fifth generation) offered both the 2.5L four-cylinder and the 3.5L V6 engine. P0305 is valid on the V6 model but anomalous on the four-cylinder, which has only cylinders 1-4.
Identifying Your 2018 Altima's Engine
The 2018 Altima offered the 2.5L QR25DE I-4 (179 hp) on base models and the 3.5L VQ35DE V6 (270 hp) on SR and SL trims. Check your trim level or door jamb sticker to confirm which engine is installed. P0305 is valid only on the V6.
Cylinder 5 Location on the V6
On Nissan's VQ35DE V6, cylinder 5 is located on the rear bank (toward the firewall) in the center position. The engine's transverse mounting places the rear bank closer to the firewall, making cylinder 5 somewhat challenging to access.
Common Causes on the V6
Spark plug wear is common at 2018 mileage levels—plugs should be replaced around 100,000 miles. Ignition coil failure can occur at cylinder 5. Fuel injector issues can cause lean misfires. The VQ35DE is generally reliable but can develop intake manifold gasket leaks affecting individual cylinder performance.
Diagnostic Process
Confirm you have the V6 engine. Access cylinder 5's spark plug on the rear bank and inspect condition. Test the ignition coil. Check for vacuum leaks around the intake manifold. Verify fuel injector operation. Review maintenance history for spark plug replacement intervals.