ITI Mechanic Auto Electrical & Electronics (AITT Theory) common mistakes
ITI Mechanic Auto Electrical & Electronics (AITT Theory) · Understand the traps and how to avoid them.
When calculating resistance from a colour code, candidates read the bands from the wrong end, so the tolerance band is taken as the first digit and the multiplier is read as the tolerance.
Textbook diagrams always show the tolerance band on the right, but in the exam the resistor is often described in words from the other direction, and candidates apply a memorised left-to-right habit without first locating the isolated tolerance band or the wider gap.
What to do instead
Before decoding, locate the band that stands alone with a larger gap, or the gold/silver band, and declare it the tolerance band; then read the remaining bands from the opposite end. Practise by writing the sequence digit-digit-multiplier-tolerance explicitly before computing the value, and check that the result is a plausible E-series value.
Candidates compute total resistance of a mixed series-parallel network by adding all values or by treating parallel branches as series, giving an answer larger than the largest individual resistor.
Under time pressure the circuit is skimmed rather than redrawn, and the rule 'parallel resistance is always less than the smallest branch' is not used as a sanity check.
What to do instead
Redraw the network on rough paper, label nodes, and reduce it in stages: first combine each pure parallel group using the reciprocal formula or product-over-sum for two resistors, then add the resulting equivalents in series. Always apply the sanity check that any parallel group must be smaller than its smallest member before selecting an option.
On battery questions, candidates confuse capacity (ampere-hours) with cold cranking current, or state that a 12 V battery with four cells in series gives 12 V, and they misapply the series/parallel rules for cells.
The terms are learned as vocabulary lists without the underlying relationship, and the difference between energy storage and short-duration current delivery is not linked to a physical picture of plate area versus number of cells.
What to do instead
Anchor each term to its unit and its purpose: Ah is charge stored and governs reserve time; CCA is a current the battery can sustain for 30 s at low temperature and governs cranking. For cell arrangements, state the rule aloud: series adds voltage and keeps capacity, parallel adds capacity and keeps voltage. Work two numeric examples of each before the exam.
When diagnosing a no-crank condition, candidates jump straight to replacing the starter motor instead of working through the circuit logically: battery state of charge, terminal voltage under load, earth strap, ignition switch input, starter relay, neutral or clutch switch, solenoid, then motor.
The starter is the most visible and familiar component, and candidates recall 'it is usually the starter' from workshop folklore rather than applying a test sequence with a voltmeter.
What to do instead
Use a fixed diagnostic ladder and state it in the answer: measure battery open-circuit and cranking voltage, then perform voltage-drop tests on the positive feed and the earth return while cranking, then confirm control-side signals at the relay and solenoid. Only condemn the motor after the supply and control sides are proven good.
Candidates describe a voltage-drop test as measuring resistance with an ohmmeter on a live circuit, or they quote acceptable drops without units, and they test continuity on a circuit that is still powered.
Continuity and voltage-drop tests are both 'checking a wire', and the procedural difference between a dead-circuit resistance test and a live-circuit volt-drop test is not firmly separated in memory.
What to do instead
Memorise the distinction: resistance and continuity are measured with the circuit isolated and unpowered; voltage drop is measured in parallel with the conductor while the circuit carries its normal load current. Quote results in volts, with the typical target of a few tenths of a volt on a power feed and earth return, and note that a drop much above that indicates excessive resistance.
Candidates answer CAN bus questions by describing it as a star or ring network with a master controller, or they claim that disconnecting one module stops the whole bus, instead of describing a terminated linear bus with dominant and recessive bits.
Familiar office-network and point-to-point wiring models are unconsciously imported, and the differential pair with 120 ohm termination is remembered as a number without a topology.
What to do instead
Build a single mental picture: a twisted pair carrying CAN_H and CAN_L in parallel between all nodes, with 120 ohm terminating resistors at each physical end giving about 60 ohm measured across the pair, and arbitration by dominant bits winning over recessive. Answer topology, termination and differential signalling explicitly whenever a bus question appears.
On high-voltage safety questions, candidates list insulated gloves but omit the full sequence: verify competence and authorisation, isolate, prove the tester on a known live source, test for absence of voltage, lock and tag, wait the specified discharge time, and re-prove the tester afterwards.
Gloves and goggles are the most memorable items, so the answer stops at personal protective equipment and never reaches the live-dead-live proving procedure that makes isolation trustworthy.
What to do instead
Memorise the isolation sequence as an ordered chain and reproduce it in order: authorised person, safe shutdown, isolate, lock and tag, prove the instrument, test for dead, allow the manufacturer's discharge time, re-prove the instrument. State that a voltage tester must be proven before and after the test, and that orange cabling identifies high-voltage circuits.
Candidates treat a fuse as a device that limits current to a safe value, or they fit a higher-rated fuse to stop it blowing, rather than recognising the fuse as a one-time sacrificial link chosen to protect the cable.
The everyday experience of a fuse blowing repeatedly invites the 'bigger fuse' fix, and the protective-device role of the fuse is learned as a definition rather than as a design rule.
What to do instead
State the principle clearly: the fuse opens the circuit once and is selected to protect the wiring and components, not to regulate current, and its rating must never exceed the cable's capability. When a fuse blows, the correct response is to find the cause by measuring current and inspecting for shorts, not to increase the rating.
In ignition and sensor questions, candidates describe the ECU as switching the primary coil current on the earth side of a conventional coil but then confuse dwell with spark duration, or they describe a Hall sensor as a variable-resistance device.
Older contact-breaker and inductive-pickup knowledge is mixed with newer electronic systems, and dwell and spark duration both sound like 'how long the coil is on'.
What to do instead
Separate the terms with a timeline: dwell is the time primary current flows and energy is stored before the spark; spark duration is the burn time after the coil is switched off. Classify sensors by output type: inductive pickup generates its own AC voltage, Hall and optical sensors switch a square wave, and variable-resistance sensors need a reference voltage. Answer with the output signal type whenever a sensor is named.
Candidates answer lighting and body electrical questions with generic statements such as 'check the bulb and wiring' instead of identifying the specific circuit behaviour, for example that a flasher unit's flash rate changes with lamp load or that a relay coil and contact circuit are separate paths.
Body electrical faults are perceived as simple, so the answer is written at a broad level and never reaches the relay control versus load distinction or the resistance-dependent behaviour of a thermal flasher.
What to do instead
For every accessory question, draw the two paths: the low-current control circuit through the switch and relay coil, and the high-current load circuit through the contacts. Then name the measurable symptom: a fast flash indicates a failed lamp reducing load, a slow or no flash indicates extra resistance or a wrong lamp rating, and a relay that clicks without load operation points to the contact side.