P0420 on a Toyota Highlander
P0420 means catalyst System Efficiency Below Threshold (Bank 1). On the Highlander, a few causes come up far more than others.
What P0420 usually means on a Highlander
P0420 on a Highlander is a bank 1 catalyst-efficiency code, most often seen on the V6s that power most of these SUVs. As the original converter ages, the downstream oxygen sensor stops seeing the exhaust cleaned up and the ECU flags the cat. It's a slow, mileage-driven fault rather than a sudden failure, common on high-mileage 1MZ-FE and 3MZ-FE Highlanders. The light stays on steady with no obvious change in how the vehicle drives.
Most common causes on the Highlander
- Aged original converter
High-mileage 1MZ-FE, 3MZ-FE, and 2GR-FE Highlanders wear out the factory bank 1 cat, and efficiency declines.
- Slow downstream O2 sensor
A tired rear Denso sensor can imitate a bad cat; verify it before replacing the converter.
- Oil consumption fouling the cat
Early-2000s 1MZ/3MZ V6s prone to oil sludge or consumption can foul the catalyst substrate.
- Exhaust leak before the cat
A leaking manifold gasket or flex pipe admits air and skews the reading on older Highlanders.
- Unresolved misfire or lean run
Unburned fuel from a failing coil or a lean mixture bakes the catalyst; resolve those faults before the cat.
What P0420 costs to fix on a Highlander
A direct-fit bank 1 converter for a Highlander V6 runs a few hundred dollars, with front-bank access being the easier of the two. If the engine is a sludge-era 1MZ/3MZ still consuming oil, address that or the new cat will foul again. Confirm the rear O2 sensor and check for exhaust leaks before buying a converter.
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.
Highlander owners also ask
My older Highlander V6 has P0420 and burns some oil — related?
Quite possibly. The early 1MZ-FE and 3MZ-FE engines could develop sludge and oil consumption, and burned oil poisons the catalyst. Sort out the oil use, or the replacement cat can suffer the same fate.
Is it safe to keep driving with P0420?
The code itself can't vouch for the Highlander — P0420 is an efficiency code rather than a running fault, so you may not notice any change. But an inefficient cat means higher emissions, and you'll need it fixed to pass an emissions test. If you notice a rattle, a sulfur smell, or power loss, get it checked promptly.
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 P0420 behavior is covered on the main P0420 page. Confirm with a scan tool before buying parts.
P0420 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.