A flashing check engine light in your 2018 Toyota 4Runner combined with P0171 (System Too Lean Bank 1) indicates a severe condition requiring immediate attention. The flashing light specifically means active misfires are occurring—the lean condition is severe enough to cause incomplete combustion that could damage your catalytic converter.
Why This Is Urgent
Your 4Runner's ECM flashes the check engine light only when misfires are severe enough to risk catalytic converter damage. P0171 indicates the engine isn't receiving enough fuel for the air entering. When too lean, combustion is incomplete, causing misfires that send unburned fuel to the catalytic converter, overheating and damaging it.
The 2018 4Runner Engine
Your 2018 4Runner uses the 4.0L V6 (1GR-FE), a proven engine known for reliability. Lean conditions in this engine typically stem from external factors like vacuum leaks rather than internal engine problems. The V6's two banks each have oxygen sensors—P0171 specifically indicates bank 1.
Common Causes of Severe Lean
Vacuum leaks are the most common cause—intake manifold gasket failure, cracked vacuum lines, or loose intake connections allow unmetered air. MAF sensor contamination provides incorrect air mass readings. Failing fuel pump or clogged fuel filter reduces fuel delivery. Stuck-open PCV valve creates a vacuum leak.
Immediate Actions
Reduce engine load immediately—pull over if practical. Do not continue highway driving with a flashing light. Limited, gentle driving to a repair facility is acceptable. Listen for hissing sounds that might indicate vacuum leaks. Check that the air intake is properly connected.