P0300 on Toyota

As 4Runners accumulate miles, the catalytic converter loses efficiency and the ECU sets P0420 (Bank 1) and/or P0430 (Bank 2) when the downstream O2 sensor starts to mirror the upstream sensor. On the 4th-gen 1GR-FE, the primary cats are integrated into the exhaust manifolds, which makes them prone to cracking and internal breakdown with heat and age — a frequently searched and relatively expensive repair. Misfires, exhaust leaks, or a failing rear O2 sensor can also trip the code, so diagnosis before parts replacement matters.

Early 4th-generation 4Runners with the 4.0L 1GR-FE V6 are known for head gasket failures, often presenting as an external coolant leak near the rear of the passenger-side cylinder head. Owners report gradual coolant loss, sweet smell, overheating risk, and misfires if the leak worsens. The issue is widely discussed in owner communities and repair databases, especially on 2003-2006 trucks.

Certain 2017-2020 Toyota 86 vehicles equipped with the FA20 (4U-GSE) boxer engine are subject to a valve spring recall. The valve springs may have been manufactured with an improper surface treatment, making them susceptible to fracture. A broken valve spring can cause engine misfires, rough running, and in severe cases, contact between the valve and piston causing catastrophic engine damage. Toyota recall campaign E0G (NHTSA 17V-587) covers free valve spring replacement at dealers.

The water pump on the 3.5L 2GR-FE fails prematurely between 80,000-120,000 miles, developing a coolant leak from the weep hole or gasket surface. If not addressed, complete pump failure causes overheating and potential head gasket damage.

The in-tank low-pressure fuel pump supplied by Denso uses a resin impeller that was molded with too low a density. The impeller absorbs gasoline, swells and deforms, then cracks and contacts the pump housing, which stops the pump. On the XX50 Avalon this produces a rough-running engine, warning lights, and eventually a stall at speed with a no-restart condition. NHTSA campaign 20V-012 (Toyota campaign 20TA02) covered the 2019 Avalon and was later expanded to additional model years and models; the overall Denso pump action grew to cover roughly 696,000 Toyota and Lexus vehicles at first and millions across successive expansions. Owners report the failure comes on suddenly rather than after a long warning period.

Toyota T-SB-0010-24 applies to 2018-2024 Camry vehicles with the A25A-FKS engine that misfire after a cold soak when engine-coolant temperature in freeze-frame data is 14-41 degrees F. The bulletin identifies P030027, P030000, P030100, P030200, P030300, and P030400. It does not authorize lash-adjuster replacement from a rough-start complaint alone: compression, spark plugs and coils, direct- and port-injector resistance, and the fuel system must first meet specification.

The 2.2L 5S-FE in the Celica GT and GT-S (shared with the Camry) is prone to head gasket failure as the cooling system ages — a marginal radiator, weak fans, or a stuck thermostat lets the iron-block/aluminum-head engine overheat, which warps the head and breaches the gasket. Once breached, coolant enters the cylinders or mixes with oil, and continued driving can ruin the head or bottom end. This is the most common major repair documented for 1990s 2.2L Toyota Celicas and Camrys.

The 1ZZ-FE engine used in many 2000-2008 Corollas is widely known for developing excessive oil consumption as piston oil-control rings stick or wear. Owners report needing to add oil between changes, blue smoke on startup or acceleration, fouled plugs, and eventual catalyst damage if the oil-burning is ignored. Toyota issued service information and owner complaints are abundant across NHTSA and enthusiast forums.

The individual coil-on-plug ignition coils fail from age and heat cycling on high-mileage Corollas, causing a cylinder misfire. This sets P0351-P0354 (coil circuit) and/or P0300-P0304 (misfire). OEM coil PN 90919-02239 is the common part for 2000-2008 1ZZ-FE cars. A failing coil produces rough idle, a flashing check engine light under load, and a raw-fuel smell; a chronic misfire can also foul the catalytic converter.

2003-2008 Corollas with the 1ZZ-FE commonly develop a vacuum leak at the intake manifold gasket, especially in cold weather. The original gasket can shrink or harden, causing rough idle, stumbling after startup, and lean-condition fault codes until the engine warms up. This issue is well known in Toyota service literature and owner communities.

On high-mileage 4A-FE/7A-FE engines the head gasket is one of the most frequently reported major failures, usually precipitated by a prior overheat event (often from a neglected cooling system, failing radiator, or stuck thermostat) rather than a design defect. Failure lets coolant migrate into the cylinders or oil galleries. Owners report gradual unexplained coolant loss with no external puddle, white/gray exhaust steam, and milky oil on the dipstick and filler cap. Because the aluminum head warps when overheated, a proper repair includes checking the head for flatness.

The 1.6L G16E-GTS turbocharged 3-cylinder engine can develop head gasket seepage under sustained high-boost driving, particularly on track or with aggressive tuning. The high specific output (300hp from 1.6L) stresses the head gasket, especially when combined with heat soak.

Toyota/Subaru recalled early 2022-2023 GR86 and BRZ models for improperly manufactured valve springs in the FA24 engine. Defective springs can fracture, causing engine misfires, rough running, and potential engine stalling while driving. Recall 22V-879.

Highlanders develop random/multiple-cylinder misfires (P0300) plus single-cylinder misfire codes from a cluster of common causes: failing ignition coils, worn spark plugs, a failing PCV valve creating a vacuum leak, and upper intake plenum gasket leaks (especially after a spark-plug job where the intake was disturbed). On the D-4S 2GR-FKS (2017+), carbon buildup on the direct injectors and intake valves can also disrupt the spray pattern and cause misfires. The rough idle and stumble are among the most-searched Highlander complaints. A flashing CEL during a misfire risks catalytic-converter damage.

The 3.5L V6 valve cover gaskets and integrated spark-plug tube seals harden and leak with age. Externally the oil weeps and bakes on the block; internally, oil seeps down the spark-plug tubes and pools around the coil-on-plug boots, fouling the plugs and corroding the coils and causing misfires. The rear (Bank 1, firewall side) valve cover is labor-intensive because the intake manifold must be removed for access, so the leak is often ignored until it causes running issues.

The 2001-2007 Highlander with the 2.4L 2AZ-FE 4-cylinder engine suffers from a design flaw where the aluminum head bolt threads strip out of the engine block. The plastic intake manifold traps heat against the block, causing thermal fatigue of the aluminum threads. This leads to head gasket failure, coolant leaks, and overheating. In severe cases, the stripped threads make a simple head gasket replacement impossible, requiring thread repair inserts (Time-Serts) or complete engine replacement. This is a known issue across Toyota vehicles using the 2AZ-FE engine.

The 4.5L 1FZ-FE's original head gasket design was prone to leaking between coolant passages, and the earlier 80-series years are the most affected. Any overheating event dramatically accelerates failure, and a factory 'center-bias' coolant temp gauge let many owners run near or over the overheat mark unknowingly. Symptoms are the classic head-gasket set: white smoke on cold start, coolant consumption with no external leak, an oily film in the coolant, milky residue on the oil cap/dipstick, and overheating. Toyota revised the cylinder head/ignition (coil-on-plug, crank/cam sensors) for the post-1998 engines, so the gasket concern is specifically a pre-1998 one. The engine is otherwise extremely long-lived, so a head-gasket job is usually worth doing.

The 1.8L 1ZZ-FE used in the base, XR and AWD Matrix (and the identical Pontiac Vibe) through the early first generation uses a narrow, low-tension oil-control ring package whose drain slots coke over with carbon. Once the rings stop draining, oil is pumped past them into the combustion chamber and consumption climbs to as much as one quart per 1,000 miles with no external leak and often no visible smoke. Owners commonly discover it only when the low-oil light comes on between changes, and running the engine chronically low accelerates bearing and VVT-i actuator wear. Toyota progressively revised the piston and ring design and USDM production was updated during the 2003-2005 window, with July 2005 generally cited by owners and rebuilders as the point at which the corrected parts were standard. This is engine-code specific: it is a 1ZZ-FE trait and does not describe the 2ZZ-GE XRS or the later 2ZR-FE.

The Toyota Paseo uses the 1.5L 5E-FE engine, which shares the head gasket weakness common to many small Toyota engines of this era. The gasket material degrades over time, leading to coolant leaks, overheating, and potential engine damage if not addressed. The Paseo is particularly vulnerable because many owners deferred maintenance on these economy cars. A head gasket replacement is straightforward on the 5E-FE due to its simple inline-4 layout.

The 2.4L 22R-E engine in the Toyota Pickup is prone to head gasket failure, often presenting as coolant leaking externally from the rear of the cylinder head or mixing with engine oil. The single-layer gasket used in early production deteriorates over time. Overheating episodes dramatically accelerate gasket failure. The 22R-E is otherwise extremely reliable, and head gasket replacement extends engine life significantly. A multi-layer steel (MLS) gasket upgrade is recommended during repair.

The supercharged (SC) version of the Toyota Previa uses a 2TZ-FZE engine with a factory supercharger. The added boost pressure accelerates head gasket wear, and failures are common at 100,000-150,000 miles. The mid-mounted engine location makes the repair extremely labor-intensive, as the engine sits under the front passenger seat area. The supercharger itself adds complexity and heat that the gasket was not well-designed to handle long-term.

The 2TZ's distributor-based ignition is a common source of rough running as the van ages. Spark plug wires crack and corrode, and the cap and rotor burn/wear, causing misfires and rough idle. A distinctive Previa failure is oil weeping past the internal distributor oil seal, which contaminates the cap/rotor and worsens misfires; a reseal kit is available. Persistent multi-cylinder weak spark can also point to the igniter/ignition control module and harness.

The 2.4L 2TZ mid-mounted four-cylinder is prone to head gasket failure, and the risk climbs on the 1994-1997 supercharged 2TZ-FZE where added boost/heat accelerates wear. Failures cluster around 100,000-150,000 miles and are almost always triggered or worsened by the Previa's marginal, hard-to-service cooling system letting the engine overheat and warp the aluminum head (aluminum head on a cast-iron block expands faster and loses the seal when it overheats). Because the engine sits under the front seats, this is one of the most labor-intensive head gasket jobs of any 1990s vehicle, which is why owners treat cooling-system upkeep as head-gasket insurance.

Gen 3 and Gen 4 Prius models develop heavy carbon buildup in the EGR valve and intake manifold, causing rough idle, misfires, and reduced performance. The Atkinson-cycle engine's low combustion temperatures promote carbon accumulation in the EGR system.

The 1.5L 1NZ-FXE engine can develop head gasket failure allowing coolant and oil mixing, often linked to EGR cooler problems that cause localized hot spots. Typically appears after 150,000+ miles. External coolant leaks may also occur at the head gasket mating surface.

The 2.4L 2AZ-FE engine used in the Solara (shared with Camry and RAV4) has a well-documented excessive oil consumption issue. The piston rings do not maintain adequate tension against the cylinder walls, allowing oil to pass into the combustion chambers. Consumption of 1 quart per 1,000-1,500 miles is common. Toyota issued a Limited Service Campaign (LSC ZE7) for some Camry models but Solara coverage has been inconsistent. Running low on oil causes catalytic converter damage and potential engine seizure.

The 3.4L 5VZ-FE V6 engine in 1995-1998 T100 trucks can develop head gasket leaks, typically presenting as external coolant leaks from the rear of the passenger-side cylinder head. The gasket material deteriorates over time, especially with infrequent coolant changes. While the 5VZ-FE is generally a reliable engine, the head gasket is its most common failure point. The V6 configuration makes the rear head gasket particularly labor-intensive to access.

The first model years of the 4.0L 1GR-FE V6 used an early head gasket design whose sealing material between the cylinder and the coolant jacket degrades over time, allowing a small coolant leak into a cylinder. The classic signature is a cold-start misfire on cylinder 4 (and sometimes cylinder 6) that 'steam-cleans' the spark plug, throwing P0304/P0306, together with slow coolant loss from the reservoir (owners report topping off every few weeks) and a gurgling sound from the heater core on startup. Because the leak is small early on, it is frequently misdiagnosed as a bad coil or plug before the head gasket is identified. Left unaddressed it progresses to overheating and possible head/block damage.

Tercels from 1991-1999 experience distributor failures at high mileage. The internal ignition coil and pickup coil degrade over time, causing intermittent no-start conditions, misfires, and stalling. The distributor cap and rotor also wear, but the internal electronics are the primary failure point. Moisture intrusion through cracked distributor caps accelerates failure.

The Toyota Tercel equipped with the 1.5L 3E-E and 5E-FE engines is prone to head gasket failure, particularly at higher mileages. The gasket material deteriorates over time, allowing coolant to leak externally or mix with engine oil. Overheating episodes accelerate gasket failure. The 5E-FE (1995-1999) is somewhat more reliable but still susceptible. Regular cooling system maintenance and avoiding overheating are key to longevity.

The 1NZ-FE/2NZ-FE coil-on-plug packs are a recurring failure point, and cylinder 1 - the one closest to the front of the engine and nearest the cowl drain area - fails disproportionately. Two mechanisms show up on the forums: the coil's internal windings break down with heat and age, and water gets down the plug tube (from a leaking valve cover gasket, a clogged cowl drain, or an engine bay wash) and sits on the coil boot, shorting the spark to ground. Owners describe replacing the plug and coil, getting a clean year, then having it recur if the water path was never fixed. A dying coil that is left alone dumps raw fuel into the catalyst and can eventually kill the converter.