Fault diagnosis, testing, repair and verification procedures
ECUs, Sensors, Actuators and Vehicle Networks
Inputs, processing and outputs
An ECU reads inputs, applies a strategy and drives outputs. Inputs include analogue voltages (potentiometers, thermistors, MAP, MAF), digital signals (switches, Hall and inductive sensors) and bus messages. A regulated 5 V reference is often shared by several sensors, so a single shorted sensor can pull every reading down - a classic exam trap. Outputs are solenoids, relays, motors and lamps, increasingly driven by PWM so current can be controlled smoothly.
Sensor types to recognise: NTC coolant and air temperature sensors; potentiometer throttle and pedal sensors (often duplicated and inverse-tracking for plausibility); Hall-effect and inductive crank/cam sensors; wideband oxygen sensors with a pump cell; knock sensors (piezo); and wheel-speed sensors. Actuators: injectors, idle and EGR valves, turbo vanes, throttle motors and relays.
Networks
CAN is a twisted, differential bus: idle is nominally 2.5 V on both lines, dominant states drive CAN-H up and CAN-L down, and two 120 Ω terminating resistors in parallel give about 60 Ω at the DLC with power off. LIN is a single-wire, single-master, low-speed bus for body devices. Others include FlexRay, MOST, automotive Ethernet and SENT. Bus faults split into short-to-power, short-to-ground, open circuit and crossed pairs - a scope shows the waveform shape that a DMM cannot.
Traps. Reading 5 V reference and ground but condemning the sensor without a load test; checking CAN resistance with the battery connected; using an ohmmeter on a live circuit; and misreading a communication DTC as a faulty module rather than a network fault.
Rule of thumb. Confirm every sensor has its reference, its earth and a plausible signal under real conditions before replacing it.
Fault Diagnosis, Testing, Repair and Verification
A disciplined process
Strong candidates follow a repeatable sequence: (1) verify the complaint and its conditions; (2) gather information - customer interview, wiring diagram, service data, DTCs and freeze-frame; (3) analyse the circuit and form a hypothesis; (4) test to confirm or eliminate it; (5) repair the root cause; (6) verify the repair and record it.
Diagnosis is a logical narrowing, not a parts lottery. A DTC names a system with a failure type, not a guilty component: a misfire code does not prove a coil, a circuit code does not prove the sensor. Use bi-directional controls to command actuators, look at live data against expected values, and compare with a known-good signal where possible.
The most powerful routine test is voltage drop under load. Measure from the supply to the load and from the load to earth while the circuit is working; anything approaching 0.5 V on a leg suggests resistance at a connection, cable or earth. Resistance checks on unloaded circuits often read fine. Current ramping on a scope shows a motor's health; a stalled or shorted motor shows high or erratic current.
Repair and verification
Repairs must restore both function and reliability: correct gauge wire, sealed splices, correct terminals, correct fuse rating, correct routing. Verification means clearing codes, running the system through its full range, test-driving to set monitors, confirming no warning lamps remain, and checking that nothing else was disturbed.
Traps. Firing the parts cannon; clearing codes without recording them; skipping the road test and readiness monitors; assuming an intermittent fault is fixed because it has not recurred; and failing to document the customer's actual complaint.
Rule of thumb. Roughly 3% of system voltage - about 0.3–0.4 V on a 12 V circuit - is the maximum acceptable total voltage drop.