Car wiring diagrams & trouble codes,
made simple.
Simple diagrams with pin numbers and real wire colours — plus easy explanations for every DTC code.
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All diagrams →Parking Assist (Sonar Sensors & Rear Camera) Wiring — Toyota Prius
Pinout of the Parking Assist ECU: power, sonar switch, rear ultrasonic sensors, rear camera, CAN and the screen link.
CAN Bus Basics — CAN-H, CAN-L and the 60 Ω Test
How car computers talk on two wires, and how to test the CAN bus with a multimeter in 2 minutes.
Alternator Charging Circuit (B, IG, S, L) — Battery Light Explained
How the alternator charges the battery, what terminals B, IG, S and L do, and how to test charging.
Starter Motor Circuit — How It Works & How to Test
Why the car only clicks or won't crank: battery cable, solenoid, relay and safety switch explained.
Relay Pinout 30, 85, 86, 87, 87a — How a Car Relay Works
What each relay pin does, how to wire it and how to test a 4- or 5-pin relay.
OBD2 Port Pinout — All 16 Pins Explained
Every pin of the OBD2 diagnostic socket: power, ground, CAN and K-line, with simple tests.
Trouble codes
All codes →P0128 — Coolant Thermostat (Below Regulating Temperature)
The engine is taking too long to warm up.
PowertrainmediumP0171 — System Too Lean (Bank 1)
The engine computer is adding a lot of extra fuel — too much air or too little fuel.
PowertrainhighP0300 — Random/Multiple Cylinder Misfire Detected
More than one cylinder is misfiring, at random.
PowertrainhighP0301 — Cylinder 1 Misfire Detected
Cylinder 1 is not burning fuel properly.
PowertrainhighP0335 — Crankshaft Position Sensor "A" Circuit
The computer is not getting a good signal from the crankshaft sensor.
PowertrainmediumP0420 — Catalyst System Efficiency Below Threshold (Bank 1)
The oxygen sensor after the catalytic converter reads almost the same as the one before it, so the converter is not cleaning the smoke well.