What’s Under the Hood in a Car

Under the hood you’ll find the vehicle’s power source (an internal combustion engine, a hybrid system, or an electric drivetrain’s power electronics), plus the supporting systems that make it run: air intake and fuel delivery, cooling and lubrication, electrical charging and control modules, brake hydraulics, and various reservoirs, belts, and sensors. While layouts vary by model, most cars place key service points—like the oil fill cap, dipsticks, coolant and brake fluid reservoirs—within easy reach at the front of the engine bay.

The Core: Engine or Electric Drive

In gasoline or diesel cars, the engine is the centerpiece. It converts fuel into mechanical power through combustion in cylinders, with a crankshaft sending torque to the transmission. You’ll often see a plastic engine cover on top; beneath it are components such as the valve cover, ignition coils, fuel injectors, and the intake manifold.

Hybrid vehicles pair a combustion engine with one or more electric motors and a power-split transmission, plus an inverter/converter unit that manages high-voltage energy. Fully electric vehicles (EVs) usually don’t have a traditional engine; instead, you’ll find power electronics (inverter, DC-DC converter, coolant manifolds) and sometimes an accessible “frunk.” In many front-wheel-drive cars, the power unit is mounted transversely (sideways); in rear- and all-wheel-drive performance cars, it’s often longitudinal (front-to-back).

Major Components You’ll Typically See

Most modern engine bays share a common set of visible parts. The items below are frequently present in gasoline vehicles and many hybrids, though exact placement and appearance differ by make and model.

• Engine assembly: Often covered by a plastic shroud; includes cylinder head/valve cover and ignition coils.
• Air intake system: Air filter box, intake duct, and mass-airflow or manifold-pressure sensor; throttle body on gasoline engines.
• Battery (12-volt): Powers starting, accessories, and control modules; sometimes located in trunk but connected via jump posts under the hood.
• Alternator or DC/DC converter: Alternator charges the 12V system on combustion cars; hybrids/EVs use a DC/DC converter.
• Serpentine belt and pulleys: Drive accessories like alternator, water pump (if belt-driven), and AC compressor.
• Fuse/relay box: Houses fuses and relays; typically labeled.
• Radiator and cooling fans: At the front, behind the grille; dissipate engine or inverter/motor heat.
• Coolant reservoir (expansion tank): Translucent plastic tank with MAX/MIN marks.
• Engine oil fill cap and dipstick: For adding oil and checking level (some newer cars use electronic level sensors instead).
• Transmission dipstick (if equipped): Many modern automatics are “sealed” and lack a user-serviceable dipstick.
• Brake fluid reservoir: On or near the brake master cylinder, typically at the rear of the engine bay on the driver’s side.
• Power steering fluid reservoir (if hydraulic): Many newer cars use electric power steering and have no fluid reservoir.
• Windshield washer fluid reservoir: Usually a blue or black cap with a windshield icon.
• AC compressor and refrigerant lines: Aluminum lines leading to compressor; EVs and some hybrids may use heat pump components.
• Electronic control units (ECUs): Engine control module and others; often in protective housings.
• ABS/ESC module: Brake hydraulic modulator with multiple steel lines.
• Turbocharger/intercooler (if equipped): Turbo is typically mounted near the exhaust side; an intercooler sits up front or on top to cool intake air.

Taken together, these components manage air and fuel flow, generate power, keep temperatures in check, store and distribute electricity, and provide braking and climate comfort.

Fluids You’ll Find Under the Hood

Fluids are the lifeblood of a vehicle. The following are the common fluids accessible in most engine bays, along with what they do and typical visual cues. Always consult the owner’s manual for the correct specification and service interval.

• Engine oil: Lubricates and cools internal engine parts; checked via dipstick or electronic monitor. Oil color darkens with use and is not a sole indicator of condition.
• Coolant/antifreeze: Circulates through engine and radiator (and hybrid/EV power electronics) to manage temperature; typically pink, orange, blue, or green depending on chemistry.
• Brake fluid: Transmits hydraulic pressure to wheel brakes; usually clear to light amber; turns darker as it absorbs moisture; use DOT spec specified by manufacturer.
• Power steering fluid (if applicable): Hydraulic assist fluid; many modern vehicles delete this in favor of electric steering.
• Windshield washer fluid: For cleaning the windshield; often blue; safe to top up at home.
• Transmission fluid (if serviceable): Lubricates and cools the transmission; access varies widely; many modern units require professional service.

Monitoring levels and leaks can prevent costly damage. Note that fluid colors vary; always identify fluids by reservoir label and manual, not color alone.

Air, Fuel, and Emissions Systems

Air Path

Ambient air enters through the grille or a fender duct into the airbox, where a filter removes debris. Sensors measure flow and temperature, and a throttle body meters intake air (on gasoline engines). Turbocharged engines compress intake air for higher power; an intercooler reduces its temperature before it enters the intake manifold.

Fuel Delivery

Modern engines use electronic fuel injection. A low-pressure pump in the tank supplies fuel; direct-injection engines add a high-pressure pump on the engine to feed injectors seated in the cylinder head. Hybrids and many modern engines use precise injector timing to balance power, emissions, and fuel economy.

Emissions Controls

Under-hood systems such as EGR valves, PCV circuits, and EVAP purge valves reduce smog-forming emissions and capture fuel vapors. Catalytic converters are typically mounted in the exhaust path close to the engine or under the floor to heat quickly and reduce pollutants.

Electrical and Control Systems

The engine control unit (ECU) manages spark timing, fuel delivery, throttle position, variable valve timing, and emissions hardware, reading from sensors like oxygen sensors, cam/crank position, knock, and temperature/pressure sensors. A 12-volt battery powers cranking and electronics; the alternator or DC-DC converter (in hybrids/EVs) keeps it charged. Multiple control modules communicate over CAN or similar networks. The OBD-II diagnostic port sits inside the cabin, usually beneath the dashboard.

Cooling and Climate Systems

The cooling system includes the radiator, water pump (belt-driven or electric), thermostat, coolant hoses, and radiator fans. Heater core lines bring warm coolant into the cabin for heat. The air-conditioning system comprises a compressor, condenser, expansion device, and evaporator; in many EVs and some hybrids, a heat pump can both heat and cool efficiently and may share coolant loops with the battery and power electronics. Many EVs have multiple dedicated coolant circuits with electric pumps and valves to optimize temperatures.

Hybrid and EV-Specific Under-Hood Layouts

Electrified vehicles replace or supplement the engine with high-voltage systems. Visibility and labeling are designed for safety, often with orange insulation indicating high-voltage lines. Here are common elements you may see in the front compartment of hybrids and EVs:

• Inverter/converter: Converts battery DC to AC for the motor and manages regenerative braking power back to DC.
• DC-DC converter: Steps high-voltage down to 12V to power accessories and charge the 12V battery.
• On-board charger (OBC): Converts AC from a wall charger to DC for the drive battery; often packaged near the front.
• Electric motor or e-axle (hybrids may integrate motor into the transmission): Provides drive torque and regen braking.
• High-voltage junction box and orange cables: Clearly marked; do not touch or service without proper training.
• Thermal management manifolds, pumps, and reservoirs: Separate coolant loops for battery, motor, and cabin heat pump.
• 12-volt battery: Still present to power control modules, locks, lights, and failsafe functions.

Because of this packaging, EVs sometimes offer extra storage space (a frunk), while hybrids maintain many traditional under-hood service points alongside power electronics.

What You Can Safely Check as an Owner

Basic inspections can improve reliability and catch small issues early. Always follow the owner’s manual and ensure the engine is cool before opening any caps.

• Washer fluid level: Top up as needed.
• Coolant reservoir level: Check against MIN/MAX when cold; never open a hot system.
• Engine oil level: Use the dipstick if equipped; add only the specified oil type.
• Brake fluid level: Should remain near MAX; a sudden drop warrants immediate inspection.
• Battery condition: Look for corrosion on terminals; ensure clamps are tight.
• Air filter: Inspect for clogging if the airbox is easily accessible.
• Belts and hoses: Check for cracks, fraying, or leaks; listen for squeals that may indicate belt or pulley issues.
• Leaks or odors: Sweet smell can indicate coolant; fuel smell requires urgent attention.

If anything seems off—warning lights, noises, visible leaks—schedule professional service rather than guessing.

Red Flags That Warrant Service

Certain signs suggest immediate attention to prevent breakdowns or safety risks.

• Overheating gauge or steam from the front of the car.
• Oil pressure warning, battery/charging light, or brake warning light illuminated.
• Bright orange cables damaged or exposed (hybrids/EVs)—do not touch; tow to service.
• Visible fuel leaks or strong fuel smell.
• Rapid coolant loss, milky oil, or mixing of fluids.
• Persistent knocking, metal-on-metal sounds, or severe belt squeal.

Addressing these issues promptly can prevent catastrophic engine or electrical damage and maintain safety.

Safety Tips Around the Engine Bay

Engine bays contain hot parts, spinning belts, and high voltage. A few precautions reduce risk when inspecting or topping up fluids.

• Turn the engine off and let it cool before touching components.
• Keep hands, hair, and clothing away from moving belts and fans (which can run even with the engine off).
• Never open a hot radiator or pressurized coolant cap.
• Avoid touching orange high-voltage cables or power electronics on hybrids/EVs.
• Use only manufacturer-specified fluids and fill points.
• Disconnect the 12V battery negative terminal before certain DIY work; consult the manual.

When in doubt, don’t poke around—modern systems are complex, and incorrect handling can be dangerous.

Why Layouts Differ

Automakers optimize under-hood design for space, safety, cost, performance, and serviceability. Front-wheel-drive cars pack the engine and transaxle sideways to save space; performance and truck platforms often mount engines longitudinally for balance and towing. Turbochargers, hybrid modules, and emissions hardware further influence placement. Engine covers and undertrays reduce noise and improve aerodynamics but can hide service items.

Summary

Under the hood lives the vehicle’s power source and the systems that feed, cool, control, and protect it. In combustion cars, that means an engine with air, fuel, cooling, lubrication, and charging components; in hybrids and EVs, power electronics and high-voltage hardware take center stage. Knowing what’s where helps you perform basic checks, recognize warning signs, and speak clearly with a technician—while staying safe around hot, moving, or high-voltage parts.

What is the most stolen item from a car?

The Top 10 Items Stolen From Vehicles

• Purses and Wallets.
• GPS Devices.
• Registration Document.
• License Plates.
• Credit Cards.
• Truck Tailgates.
• Garage Door Opener.
• Stereo Equipment.

What is the plastic under the hood of a car called?

The engine splash shield, also known as the skid plate, lower engine cover, or engine splash guard, is a panel located underneath your vehicle’s engine to protect it from debris on the road.

Is the transmission under the hood?

Yes, the transmission is located under the hood on most vehicles, specifically bolted to the engine, though its exact position can vary depending on whether the car is front-wheel or rear-wheel drive. In a front-wheel drive car, the transmission is typically found directly under the hood, integrated with the engine. For rear-wheel drive vehicles, the transmission sits behind the engine, further back from the front of the car.
 
Transmission location by drivetrain type: 

• Front-Wheel Drive (FWD): Opens in new tabThe transmission is located under the hood and is part of the transaxle, a combined unit of the engine and transmission that sends power to the front wheels.
• Rear-Wheel Drive (RWD): Opens in new tabThe transmission is situated behind the engine and is often found in the middle section of the car, accessible from underneath.

Why it’s not always easily visible: 

• Even though it’s under the hood, the transmission is large and located behind other engine components, making it difficult to see.
• Many vehicles will have a separate dipstick located near the transmission for checking the transmission fluid level.

What is under the hood of the car?

Under the hood of a car, you’ll find the engine, which provides power to move the vehicle. Other key components include the battery, for electrical power; the radiator and coolant, to keep the engine cool; the air filter, to supply clean air to the engine; various fluid reservoirs like brake, power steering, and windshield washer fluid; and the fuse box for electrical protection.
 
Here is a breakdown of the main components and their functions:

• Engine: The “heart” of the car, it converts fuel into mechanical energy to power the wheels. 
• Battery: Provides the electrical energy to start the engine and power lights, radio, and other electrical systems. 
• Alternator: A generator that produces electricity while the engine is running to keep the battery charged. 
• Radiator and Coolant: The radiator is a heat exchanger that circulates engine coolant (antifreeze) to absorb engine heat and prevent overheating. 
• Air Filter: Prevents dirt and debris from entering the engine by filtering the air drawn in for combustion. 
• Brake Fluid Reservoir: Holds the fluid that transmits pressure to the brake system, allowing you to stop the car. 
• Power Steering Fluid Reservoir: Contains fluid that helps to make steering easier. 
• Windshield Washer Fluid Reservoir: Stores fluid to be sprayed on the windshield for cleaning. 
• Fuse Box: Contains fuses that protect the car’s electrical components from electrical surges. 
• ECU (Electronic Control Unit): A “smart” computer that controls many of the car’s advanced features and systems. 
• Oil Fill Cap and Dipstick: You can check the engine oil level using the dipstick and add more oil through the fill cap, located on the engine.