Component reference
These are the parts people replace speculatively, and they are expensive to guess at. Each entry covers what the part does, how it actually fails, and — the part that matters — how to test it before you buy one.
| Component | What it does | Typical cost |
|---|---|---|
| Fuel Injector | An electrically operated valve that sprays a metered, atomized quantity of fuel on command. Failures split into flow problems, spray-pattern problems, and electrical problems, and each is found a different way. | $200–$2,000 |
| Throttle Body | The valve that meters how much air the engine gets. On nearly everything built since the mid-2000s it is electronic: the pedal is a sensor and a motor moves the throttle plate. | $150–$900 |
| Component | What it does | Typical cost |
|---|---|---|
| Ignition Coil | Steps battery voltage up to the tens of thousands of volts needed to jump the spark plug gap. A misfire code names a cylinder, not a part — the coil is one of at least four things that can cause it. | $100–$1,200 |
| Component | What it does | Typical cost |
|---|---|---|
| 12-Volt Battery | A chemical store of electricity. Its real jobs are cranking the starter, keeping modules alive while the car is parked, and steadying system voltage — it is not what powers the vehicle while the engine is running. | $180–$500 |
| Alternator | The alternator generates the electricity the car runs on while the engine is turning, and recharges the battery. The battery only starts the car; the alternator does everything after that. | $350–$1,200 |
| Catalytic Converter | Converts engine-out pollutants into less harmful gases using a precious-metal-coated ceramic honeycomb. P0420 reports that the converter is not cleaning up as well as expected — it does not, by itself, prove the converter is bad. | $500–$3,000 |
| Spark Plug | Delivers the spark that ignites the air-fuel mixture, and doubles as a readable record of what is happening inside the cylinder. A genuine wear item — the gap grows continuously from the day it is installed. | $100–$900 |
| Starter Motor | An electric motor that spins the engine fast enough for it to start running on its own, combined with a solenoid that both engages the drive gear and switches the very high current on. | $300–$1,000 |
| Thermostat | A temperature-controlled valve that blocks coolant flow to the radiator until the engine reaches operating temperature. Stuck open wastes fuel; stuck closed can destroy the engine. | $150–$900 |
| Component | What it does | Typical cost |
|---|---|---|
| Fuel Pump | Delivers fuel from the tank to the injectors at a controlled pressure and volume. A pump that holds pressure at idle can still be failing — volume under load is where they die first. | $400–$1,600 |
| Component | What it does | Typical cost |
|---|---|---|
| Camshaft Position Sensor | Tells the computer where the camshaft is in its rotation. That is how the engine knows which cylinder is on its firing stroke, and it is the feedback signal for variable valve timing. | $120–$450 |
| Crankshaft Position Sensor | The engine's master timing reference. It reports exactly where the crankshaft is and how fast it is turning. On most engines, if this signal disappears the engine will crank but never start — and if it fails while you are driving, the engine can shut off at speed. | $150–$550 |
| Mass Air Flow (MAF) Sensor | Measures the actual mass of air entering the engine so the computer knows how much fuel to inject. It fails by drifting low rather than dying outright, which is why it gets blamed and replaced so often. | $150–$500 |
| Oxygen Sensor (O2 / Air-Fuel Ratio Sensor) | Measures oxygen left in the exhaust so the computer can correct how much fuel it injects. Upstream sensors control fueling; downstream sensors mostly grade the catalytic converter. | $150–$500 |
| Component | What it does | Typical cost |
|---|---|---|
| EGR Valve | A valve that routes a measured amount of exhaust back into the intake to cool combustion and cut NOx emissions. On a high-mileage engine it is one of the most common things to find packed with carbon. | $200–$1,000 |
| PCV Valve | The one-way metering valve that draws combustion gases out of the crankcase and feeds them back into the intake to be burned. A cheap part with badly disproportionate consequences when it fails. | $40–$500 |
Our database holds thousands of component identifiers pulled from the trouble-code data, but most of them are machine-generated labels rather than things worth writing an encyclopedia entry about. Only components we have actually written about appear here.
Cost ranges are US independent-shop estimates including labour, and they are wide because access varies enormously between vehicles.