Emissions, ignition, fuel, or sensor fault. Most frequent: loose gas cap (P0455), O2 sensor, MAF, catalytic converter, misfire (ignition coil/spark plug).

• Steady light – usually emissions related, car may drive normal.
• Flashing light – severe misfire, stop heavy load to avoid cat damage.
• Rough idle, hesitation, loss of power, increased fuel consumption.
• Possible smell of unburnt fuel.

• Use OBD2 scanner (Bluetooth dongle or professional). Record all codes and freeze frame.
• Check gas cap: click at least 3 times. Clear codes and drive – if returns, further diagnosis.
• Inspect air filter, vacuum hoses for cracks, Mass Air Flow sensor (clean with spray).
• Monitor live data: fuel trims, O2 sensor switching, coolant temp, calculated load.
• For misfire codes (P030x): swap coils/plugs to identify faulty cylinder.
• Check for Technical Service Bulletins (TSBs) for your 4Runner model year.

Alternator not charging, weak battery, loose belt, corroded terminals, parasitic drain.

• Red battery icon, may flicker at idle.
• Headlights dim when idling, brighten with revs.
• Radio resets, power windows slow.
• Clicking when starting or no crank.

• Multimeter: engine off 12.4V+; running 13.8–14.8V (if below 13.5V, alternator weak).
• Load test battery (auto parts store free).
• Inspect alternator belt tension, serpentine belt condition.
• Check for voltage drop on battery terminals and ground straps.
• Parasitic draw test (multimeter in series) – if >50mA, locate faulty circuit.

Low oil level, oil pump failure, clogged pickup, faulty sending unit, or wrong oil viscosity.

• Light on at idle, goes off when revved – possible worn bearings/ low pressure.
• Engine ticking, knocking, or rattling (especially on startup).
• Overheating due to insufficient lubrication.

• Immediately stop engine, check dipstick level and condition (milky? indicates coolant).
• If level low, top up and look for leaks (oil filter, drain plug, valve cover).
• Install mechanical oil pressure gauge to verify actual pressure (spec: >4 psi at idle, >30 psi at 2000rpm).
• Inspect oil pressure switch wiring – sometimes shorted.
• If pressure truly low, drop oil pan, check pickup tube screen for sludge, then oil pump.

Low coolant, stuck thermostat, failed water pump, electric fan not working, clogged radiator, head gasket leak.

• Coolant smell inside cabin (heater core).
• Steam from under hood, coolant puddles.
• Temperature gauge fluctuating or pegged hot.
• Coolant bubbling in reservoir.

• Wait for engine cool, check radiator and reservoir level – top up with correct Toyota pink coolant.
• Pressure test cooling system to find external leaks.
• Test thermostat: remove and place in hot water (should open at 180–195°F).
• Verify electric fans come on with A/C or at ~205°F; check fan relays.
• Check for exhaust gases in coolant (combustion leak tester) – indicates head gasket.

Parking brake engaged, low brake fluid, worn pads, ABS/master cylinder leak, brake booster failure.

• Red (!) light on dash, sometimes accompanied by beep.
• Soft or low brake pedal, increased travel.
• Grinding noise when braking (wear indicators).

• Release parking brake – if light stays, check switch under lever.
• Check fluid level in brake reservoir – if low, inspect pads (worn) and calipers for leaks.
• Pump pedal with engine off – if pedal firms up, suspect vacuum booster (engine running hiss).
• For ABS module issue, scan ABS codes (C0278 etc.).
• Brake fluid moisture test (tester or strips) – moisture reduces boiling point.

Wheel speed sensor failure (tone ring rust/damage), ABS pump/module fault, blown fuse, wiring harness chafing.

• Amber ABS light stays on after bulb check.
• ABS does not activate during hard braking (wheels may lock).
• Speedometer may work erratically (if rear sensor shared).

• Use ABS-capable scanner to retrieve codes (C0200, C1235 etc.).
• Visually inspect each wheel speed sensor and tone ring (remove wheel if needed).
• Measure sensor resistance: typically 1.0–2.0 kΩ. Compare left/right.
• Check AC voltage while spinning wheel (should generate small AC).
• Inspect wiring for damage near suspension components.

Usually triggered by check engine light (engine/transmission fault disables stability control). Also: steering angle sensor, yaw rate sensor, brake light switch.

• VSC light ON, sometimes “VSC OFF” or slip icon blinking.
• Traction control inactive (wheels spin freely).
• Often accompanied by Check Engine or ABS light.

• Scan ECU for DTCs – primary cause is often in engine/ABS (e.g. O2 sensor).
• Check brake lights operation – faulty switch can disable VSC.
• Perform zero-point calibration (steering angle sensor) after repairs: drive straight then turn wheel lock-to-lock.
• Inspect yaw rate sensor (under center console) for secure mounting.

4WD actuator (ADD), encoder motor on transfer case, position switch, low ATF in transfer case (on V8 models), or overheating automatic transmission (A/T Oil Temp light).

• Flashing 4WD light, unable to shift into 4Hi/4Lo.
• Grinding or clunk when engaging.
• “A/T OIL TEMP” light on – transmission overheating (stop immediately).

• Check transfer case fluid level/condition (dipstick if equipped).
• Listen for actuator motor operation when shifting – if silent, motor or wiring fault.
• Use scan tool with bidirectional control to test 4WD system.
• Inspect undercarriage wiring for damage, especially after off-road.
• For A/T temp light: check transmission fluid level, cooler lines, and radiator condition.

Low tire pressure (cold weather), dead sensor battery (life 5-7 years), sensor damage, faulty receiver module.

• Yellow TPMS light solid or flashing (flashing = sensor fault).
• Individual tire pressure not displayed on MID.
• Light comes on after tire rotation (sensor not relearned).

• Check and inflate all tires to placard pressure (usually 32-35 psi).
• Use TPMS activation tool to trigger each sensor; verify IDs with scan tool.
• If one sensor doesn’t respond, it likely needs replacement.
• Relearn procedure after sensor replacement (some 4Runners require special sequence).
• Check aftermarket wheels – may have no sensors installed.

Clockspring failure (worn ribbon), front impact sensor, seat belt pretensioner, passenger occupant sensor, or SRS module.

• Airbag light stays on or flashes a code.
• Horn or steering wheel audio controls may not work (clockspring).
• “PASSENGER AIRBAG OFF” light may be faulty.

• Use advanced scanner (SRS-capable) to read codes (B0100, B0110 etc.).
• Inspect yellow connectors under seats (often loose after car detailing).
• Check clockspring continuity with multimeter (requires removal).
• Never measure resistance on an airbag module directly – use scanner.

Lock actuator failure, position sensor, or wiring. Light blinking indicates not engaged.

• RR DIFF LOCK light flashes continuously.
• Diff doesn’t lock (no traction improvement).

• Ensure transfer case is in 4Lo (required for lock).
• Listen for actuator motor under rear of vehicle.
• Check wiring near axle for damage.

Accompanied by message on multi-information display. Can be low fuel, door ajar, maintenance reminder, or system fault (hybrid/brake).

• Check display for specific message.
• Cycle through warning screens.
• Often just a reminder – but if persists, scan all ECUs.