P0304 Toyota Highlander
Cylinder 4 Misfire Detected · Powertrain Code · High · Repair: Moderate
What is P0304?
Misfire detected in cylinder 4. Typically caused by a worn spark plug, bad ignition coil, faulty fuel injector, or low compression in that cylinder.
When the Toyota Highlander triggers code P0304 (cylinder 4 misfire detected), it typically points to an issue in the powertrain system. This is a high severity code, so address it promptly to avoid further damage or safety concerns. Below you'll find the most common causes, symptoms, and repair options specific to the Toyota Highlander.
This repair is rated moderate, meaning some experience and specialty tools required.
Diagnostic data on this page is compiled from publicly available repair resources and technical service bulletins. Always have a qualified mechanic confirm the diagnosis before authorizing repairs on your Toyota Highlander.
Common Causes on Toyota Highlander
- Cylinder 4 misfire — degraded spark plug or failed coil-on-plug unit
- Fuel injector failure on cylinder 4
- Valve clearance out of specification — Toyota recommends checking every 60k miles
- Head gasket seepage on cylinder 4 (rare but possible on high-mileage 2AZ-FE)
Symptoms
- Flashing CEL during active misfire
- Noticeable engine shake at idle
- Significant power loss
- Increased exhaust emissions and possible cat converter damage if driven
- Poor fuel economy
Notes for Toyota Highlander
On 4-cylinder Toyota engines (2AR-FE, 2ZR-FE), cylinder 4 is at the timing chain end — check for oil leaking onto the plug from the valve cover gasket.