THE WORDS
BEHIND THE TEST.

A reference voltage supplied through a circuit so a sensor can shift that voltage as conditions change. Always verify the bias source and its ground before condemning the sensor.

A resistor that holds a signal line high until a switch or open-collector output pulls it low. Common on digital sensors and LIN communication.

A resistor that holds a signal line low until a switch or output drives it high. Verify the resistor path before blaming the sensor or module.

The clean electrical reference used by a sensor or controller. A ground can pass a continuity check yet fail under load because of resistance.

The voltage lost across resistance while current flows. Measure across the connection or cable under load—not from the terminal to an assumed ground.

A measurement taken while the circuit is operating and carrying current. It reveals resistance and weak connections that an unloaded voltage check can miss.

A broken or interrupted path that prevents current flow. Locate opens with voltage checks, loaded tests and connector or harness inspection.

An unintended connection to ground or a voltage source. Identify which condition exists before replacing a fuse, driver or component.

A meter or scope input that draws very little current, allowing signal measurement without significantly loading the circuit.

Accessing a connected circuit from the rear of a connector to measure it while operating. Avoid spreading terminals or piercing sealed wires unnecessarily.

A voltage measurement taken across a cable, fuse, relay or ground while current is flowing. Excess drop identifies resistance that static checks miss.

The percentage of one repeating cycle that a signal stays active. Compare commanded and measured duty cycle under the same operating conditions.

How many cycles occur per second, measured in hertz (Hz). A changing frequency often represents speed or position.

The time a pulse remains active within each cycle. Pulse width can change independently of frequency and often represents command magnitude.

The brief current surge when a motor, lamp or capacitor is first energized. Compare it with the running current and the circuit rating.

The shape and rate at which actuator current rises. A ramp can reveal mechanical loading, binding, shorted turns or driver faults.

The voltage spike created when current through a coil is interrupted. Its shape and clamp behavior can reveal driver, winding or suppression faults.

A decaying oscillation after a fast edge or inductive event. Excessive ringing can indicate poor suppression, wiring inductance or a failing load.

The difference between the highest and lowest points of a waveform. It is useful for comparing signal amplitude and noise.

The voltage threshold that tells an oscilloscope when to begin displaying a waveform. A stable trigger makes repeating signals readable.

The background electrical variation present when the intended signal is absent or steady. Excess noise can hide dropouts and switching faults.

The resistance across a differential network measured with power off. On a correctly terminated CAN bus, two 120 Ω terminators read about 60 Ω in parallel.

Information represented by the voltage difference between two conductors. Noise picked up equally by both wires is largely rejected by the receiver.

CAN bus states: a dominant bit overrides a recessive bit when nodes transmit at the same time. This arbitration allows the highest-priority message to continue.

The resting voltage established on a communication line by its bias or termination network. Compare both wires and follow the vehicle-specific specification.

A CAN controller state reached after excessive communication errors. Find the physical-layer or wiring cause before replacing the module.

A controller that routes messages between separate vehicle networks. A gateway fault can make several unrelated systems appear unavailable.

A switch, message or scheduled task that brings a sleeping module or network back online. Correlate the event with current draw and bus activity.

The low-power state and current draw expected after modules time out. Measure after the vehicle reaches its specified sleep state.

Pending codes indicate a recent failure, confirmed codes meet the monitor criteria, and permanent codes remain until the vehicle proves the repair through drive cycles.

The operating conditions captured when a diagnostic monitor sets a code. Use it to reproduce the fault before clearing evidence.

An emissions self-test that reports whether its required drive conditions have completed. Clearing codes or disconnecting the battery resets readiness.

A monitored low-voltage circuit that confirms high-voltage connectors, covers and service plugs are secured. An open HVIL prevents or disables high-voltage operation.

A resistor-and-contactor path that gradually charges inverter capacitors before the main high-voltage contactors close. Abnormal precharge can prevent READY mode.

Resistance between the high-voltage system and chassis ground. Follow the manufacturer procedure and safety limits; never improvise a high-voltage test.

The measurement-category rating of test equipment for transient overvoltage environments. Use leads and meters rated for the vehicle circuit being tested.