P0300 on a Toyota Highlander
P0300 means random/Multiple Cylinder Misfire Detected. On the Highlander, a few causes come up far more than others.
What P0300 usually means on a Highlander
P0300 on a Highlander is a random or multi-cylinder misfire the ECU can't isolate to one cylinder. On the V6s the rear-bank plugs and coils sit under the intake plenum, so they get neglected and are common misfire sources. The 1MZ-FE and 3MZ-FE rely on aging Denso coils, and overdue plugs are a frequent trigger across the board. Expect a rough idle and a stumble under load, with a flashing light if the misfire threatens the cat.
Most common causes on the Highlander
- Worn spark plugs
The rear-bank plugs on the Highlander V6 hide under the intake and often get skipped, causing misfires.
- Aging ignition coils
Denso coil-on-plug units on the 1MZ/3MZ/2GR weaken with age and misfire across cylinders.
- Vacuum or PCV leak
A split vacuum or PCV hose leans every cylinder at once and shows up as a random misfire.
- Oil consumption on a sludge-era V6
An early 1MZ/3MZ burning oil can foul plugs and cause intermittent misfires.
- Weak fuel delivery
A tired pump or dirty injectors can starve multiple cylinders under load.
What P0300 costs to fix on a Highlander
Plugs and coils are moderate parts, but the Highlander V6's rear bank hides under the intake plenum, so labor rises for those cylinders. The 2AZ-FE four is far easier to service. Because you have to pull the intake for the rear bank anyway, it's worth doing the whole plug and coil set at once.
Fixing it
On a Highlander V6, replace the full plug set while the intake plenum is off rather than reaching in twice, and swap a suspect coil to another cylinder to see whether the misfire follows.
For the full step-by-step diagnosis order (any vehicle), see the main P0300 guide.
Year, engine and mileage change the answer. Start a case and Redline weighs these causes against what your car shows, then names the check that would settle it.
Other codes on the Toyota Highlander
Related symptoms
Toyota Highlander problems & recalls by year
Based on widely reported patterns for this model; individual causes vary by year, engine and history. Generic P0300 behavior is covered on the main P0300 page. Confirm with a scan tool before buying parts.
P0300 on your Highlander?
Every problem becomes a Case. Tell Redline what your Highlander is doing: it shows what the evidence supports, what’s still missing, and what to check next, before you approve a repair.
“Inner pad ~1 mm. Outer pad 3 mm.”
Next: check the slide pins move freely before buying parts.