The 2021 4Runner with the 4.0L V6 (1GR-FE) is a generally solid truck, but the frequent transmission oil cooler failures and well-documented secondary air injection issues can bite owners hard. Engine rebuild frequency in the data suggests either premature wear from deferred maintenance or aftermarket abuse.
Transmission Oil Cooler Failure / Transmission Contamination
Common · high severityTypical onset: 60,000-120,000 mi
Symptoms: Milky or pink transmission fluid (coolant mixing), Transmission slipping, delayed shifts, or limp mode, Overheating transmission temp gauge reading
Fix: Replace external cooler and flush system if caught early (2-3 hrs). If coolant entered transmission, full rebuild or replacement required (12-18 hrs labor). This is a known Toyota weak point across multiple platforms.
Estimated cost: $800-6,500
Secondary Air Injection System Failure (P0410, P0418, P0419)
Common · low severityTypical onset: 70,000-110,000 mi
Symptoms: Check engine light with P0410/P0418/P0419 codes, Rattling or buzzing noise on cold start for 60-90 seconds, No drivability issues, emissions test failure in strict states
Fix: Air pump or one-way valves fail. Pump replacement 2 hrs, valve replacement 1.5 hrs. Many owners delete the system entirely if emissions testing allows (1 hr plus tune).
Estimated cost: $600-1,400
Front Lower Ball Joint Wear
Occasional · medium severityTypical onset: 80,000-130,000 mi
Symptoms: Clunking over bumps at low speed, Wandering steering or loose feel, Uneven tire wear on inside edge
Fix: Ball joints are pressed into lower control arms. Replace both sides (alignment required). 3-4 hrs labor. Aftermarket heavy-duty options available for off-road use.
Estimated cost: $700-1,200
Exhaust Manifold Leak / Cracking (Driver Side)
Occasional · low severityTypical onset: 90,000-150,000 mi
Symptoms: Ticking or tapping noise from engine bay on cold start, Smell of exhaust in cabin with heat on, Visible soot around manifold gasket area
Fix: Driver-side manifold cracks or gasket fails. Manifold replacement is 5-6 hrs due to tight clearances. Gasket-only job if caught early is 3 hrs.
Estimated cost: $800-1,800
Premature Piston Ring Wear / Oil Consumption (Abused or Neglected Units)
Rare · high severityTypical onset: 100,000-180,000 mi
Symptoms: Burning more than 1 quart per 1,000 miles, Blue smoke on deceleration or startup, Low compression on cylinder leak-down test
Fix: The 1GR-FE is typically bulletproof, but extended oil changes or aggressive off-road use without proper cooling can cause premature ring wear. Engine rebuild or short-block replacement: 18-24 hrs labor.
Estimated cost: $4,500-8,000
Radiator Plastic Tank Cracking
Occasional · medium severityTypical onset: 100,000-140,000 mi
Symptoms: Coolant leak from radiator end tank, Overheating under load or in traffic, Visible coolant pooling under vehicle
Fix: Plastic end tanks crack with age and heat cycles. Radiator replacement is straightforward: 2 hrs labor. Use OEM or quality all-aluminum aftermarket.
Estimated cost: $400-800
Fuel Pump / Fuel Filter Issues (Post-Contaminated Fuel)
Rare · medium severitySymptoms: Hard starting after sitting, Sputtering or stumbling under acceleration, Check engine light with fuel trim codes
Fix: In-tank pump and filter are combined. Failure is rare unless bad fuel introduced. Drop tank, replace pump assembly: 3 hrs labor.
Estimated cost: $600-1,100
Buy it used—the 4Runner is dead-reliable if maintained, just budget $1,500 for the inevitable trans cooler and secondary air repairs around 80k miles.
AI-assisted summary drawn from NHTSA recall data, our labor-times database, and platform knowledge. Not a substitute for a pre-purchase inspection on a specific vehicle.