What should this be reading?

Display symbols and what they mean

• OL

  Reads

  Over limit

  It means

  Resistance too high to measure — the path is open

  Usual cause

  Broken wire, blown fuse, open coil, probes not touching

• 1. (blinking, left of screen)

  Reads

  Same as OL on older meters

  It means

  Over range

  Usual cause

  Manual-range meter set too low, or an open circuit

• 0.00 Ω

  Reads

  Effectively no resistance

  It means

  A complete, healthy path

  Usual cause

  Good cable, closed switch, shorted probes

• 0.00 V

  Reads

  No potential difference

  It means

  Either no supply, or both probes on the same point electrically

  Usual cause

  Open feed, blown fuse, or a black probe on the same wire as the red

• Minus sign (-12.4 V)

  Reads

  Polarity is reversed

  It means

  Red is on the more negative point

  Usual cause

  Probes swapped, or you are back-probing an earth-switched circuit

• mV instead of V

  Reads

  Millivolts — a thousandth of a volt

  It means

  Autorange dropped down because the reading is tiny

  Usual cause

  Voltage drop testing, sensor earth checks

• Reading drifts then settles

  Reads

  Autorange hunting

  It means

  The meter is choosing a range

  Usual cause

  Normal on the first second of any measurement

• Ghost voltage (2–8 V, floating)

  Reads

  Induced voltage on an open wire

  It means

  High input impedance meter picking up nearby fields

  Usual cause

  Disconnected wire lying alongside a live harness

• Continuity beep

  Reads

  Below roughly 30–50 Ω on most meters

  It means

  The meter thinks it is a connection

  Usual cause

  Set by the meter, not by the circuit

• nF / µF

  Reads

  Capacitance

  It means

  Nano and microfarads

  Usual cause

  Suppression capacitors, sensor testing

• AC on a DC circuit

  Reads

  Alternating component present

  It means

  Meter set to V~ or AC

  Usual cause

  Alternator ripple, inductive sensor output

Probe lead resistance is typically 0.1–0.3 Ω. Short your probes together first and subtract that figure from any low-resistance reading.

Components

• Meter leads shorted together

  Healthy reading

  0.1 – 0.3 Ω

  Fault reading

  Above 0.5 Ω

  Notes

  Zero the meter or subtract this figure

• Good earth strap / bonding lead

  Healthy reading

  Under 0.2 Ω

  Fault reading

  Above 0.5 Ω, or OL

  Notes

  Measure strap only, both ends off

• Blade fuse

  Healthy reading

  Under 0.1 Ω

  Fault reading

  OL

  Notes

  Out of the box, not in circuit

• Relay coil (pins 85 / 86)

  Healthy reading

  60 – 120 Ω

  Fault reading

  OL or under 30 Ω

  Notes

  Standard 12 V ISO mini relay

• Relay contacts, energised (30 / 87)

  Healthy reading

  Under 0.2 Ω

  Fault reading

  Above 1 Ω or intermittent

  Notes

  Tap the relay while measuring

• Injector — saturated (port petrol / diesel low-side)

  Healthy reading

  11 – 17 Ω

  Fault reading

  Outside range or OL

  Notes

  Compare all cylinders at the same temperature

• Injector — peak and hold

  Healthy reading

  0.8 – 3 Ω

  Fault reading

  OL or near zero

  Notes

  Driven by a current-controlled driver

• Piezo injector (common rail)

  Healthy reading

  Capacitance test only

  Fault reading

  n/a

  Notes

  Typically 1.5 – 4 µF

• Ignition coil — primary

  Healthy reading

  0.4 – 2 Ω

  Fault reading

  OL or under 0.2 Ω

  Notes

  Coil-on-plug and pack types

• Ignition coil — secondary

  Healthy reading

  5 – 15 kΩ

  Fault reading

  OL or wildly low

  Notes

  Not measurable on many COP designs

• Glow plug

  Healthy reading

  0.5 – 2 Ω cold

  Fault reading

  OL

  Notes

  Measure plug to engine block

• Starter motor field / armature

  Healthy reading

  Under 1 Ω

  Fault reading

  OL

  Notes

  Best judged by current draw, not ohms

• Blower motor

  Healthy reading

  0.5 – 3 Ω

  Fault reading

  OL or near zero

  Notes

  Rises as brushes wear

• Blower resistor pack

  Healthy reading

  0.3 – 3 Ω per tap

  Fault reading

  OL on one or more taps

  Notes

  Speeds fail one at a time

• Speaker

  Healthy reading

  3 – 8 Ω

  Fault reading

  OL or under 1 Ω

  Notes

  Nominal 4 Ω is typical

• ABS / wheel speed sensor (inductive)

  Healthy reading

  800 – 2000 Ω

  Fault reading

  OL or shorted to earth

  Notes

  Compare corner to corner

• HV interlock loop (hybrid / EV)

  Healthy reading

  Under 5 Ω closed

  Fault reading

  OL

  Notes

  System de-energised and proven dead first

Always measure components disconnected. In-circuit readings include every parallel path in the harness.

Temperature sensors (NTC thermistor) — resistance falls as it warms

• -10 °C

  Coolant / intake temp sensor

  9 – 11 kΩ

  Typical signal voltage

  4.0 – 4.5 V

  Reading if wrong

  Rich running, hard hot start

• 0 °C

  Coolant / intake temp sensor

  5 – 6 kΩ

  Typical signal voltage

  3.8 – 4.2 V

  Reading if wrong

  Rich, high idle held too long

• 20 °C (cold soak)

  Coolant / intake temp sensor

  2.2 – 2.8 kΩ

  Typical signal voltage

  3.0 – 3.5 V

  Reading if wrong

  Should equal ambient after overnight

• 40 °C

  Coolant / intake temp sensor

  1.1 – 1.3 kΩ

  Typical signal voltage

  2.2 – 2.6 V

  Reading if wrong

  Warm-up phase

• 80 °C (normal running)

  Coolant / intake temp sensor

  300 – 380 Ω

  Typical signal voltage

  1.0 – 1.5 V

  Reading if wrong

  Fan should not be running yet

• 100 °C

  Coolant / intake temp sensor

  170 – 200 Ω

  Typical signal voltage

  0.5 – 0.9 V

  Reading if wrong

  Fans on, warning likely

Figures are for a common 2.5 kΩ-at-25 °C NTC with a 5 V reference and pull-up resistor. Manufacturer curves vary — use the shape of the curve, not the exact number.

The three wires behind almost every sensor

• 5 V reference (VREF)

  Key on, engine off

  4.9 – 5.1 V

  Engine running

  4.9 – 5.1 V

  If it is wrong

  0 V, or dragged to 3–4 V

• Sensor earth (low reference)

  Key on, engine off

  Under 0.1 V to battery negative

  Engine running

  Under 0.1 V

  If it is wrong

  Above 0.2 V

• Signal wire, sensor unplugged

  Key on, engine off

  4.5 – 5 V (pulled up) or 0 V

  Engine running

  n/a

  If it is wrong

  Neither

• Battery feed to powered sensors

  Key on, engine off

  Within 0.5 V of battery

  Engine running

  Within 0.5 V of charging voltage

  If it is wrong

  More than 0.5 V low

Common sensor signals

• MAP sensor (analogue)

  At rest / key on

  4.0 – 4.8 V (atmospheric)

  Working range

  1.0 – 1.5 V at idle, rising with load

  What a stuck value means

  Stuck near 4.5 V

• MAF — analogue voltage type

  At rest / key on

  0.6 – 1.0 V

  Working range

  1.5 – 2.5 V at idle, up to 4.5 V at full load

  What a stuck value means

  Flat 0.5 V or pegged 5 V

• MAF — frequency type

  At rest / key on

  1.9 – 2.5 kHz

  Working range

  2.5 – 7 kHz with load

  What a stuck value means

  No change with throttle

• Throttle position (TPS)

  At rest / key on

  0.5 – 0.9 V closed

  Working range

  4.0 – 4.7 V wide open

  What a stuck value means

  Dropout mid-sweep

• Accelerator pedal (APP), track 1

  At rest / key on

  0.7 – 1.0 V

  Working range

  3.8 – 4.6 V floored

  What a stuck value means

  Both tracks identical

• Accelerator pedal (APP), track 2

  At rest / key on

  0.3 – 0.5 V

  Working range

  1.9 – 2.3 V floored

  What a stuck value means

  Ratio wrong

• Crank sensor — Hall (3 wire)

  At rest / key on

  Switching 0 – 5 V or 0 – 12 V

  Working range

  Square wave while cranking

  What a stuck value means

  Flat line

• Crank sensor — inductive (2 wire)

  At rest / key on

  0 V static

  Working range

  0.5 – 5 V AC cranking, more at speed

  What a stuck value means

  Under 0.3 V AC

• Cam sensor (Hall)

  At rest / key on

  5 V or 12 V high, drops to under 1 V

  Working range

  Switching in time with the cam

  What a stuck value means

  No switch

• Narrowband oxygen sensor

  At rest / key on

  0.45 V bias, key on

  Working range

  Switching 0.1 – 0.9 V, several times a second

  What a stuck value means

  Flat 0.45 V

• Wideband (LSU) — nominal

  At rest / key on

  Scan data reads lambda 1.00

  Working range

  0.75 – 1.30 lambda

  What a stuck value means

  Pumping current pegged

• Knock sensor

  At rest / key on

  0 V

  Working range

  Millivolts AC, rises when tapped

  What a stuck value means

  No response to a light tap

• Fuel rail pressure sensor

  At rest / key on

  0.5 V key on, engine off

  Working range

  1.0 – 4.5 V with pressure

  What a stuck value means

  Stuck 0.5 V running

• Brake / clutch switch

  At rest / key on

  0 V or 12 V depending on type

  Working range

  Swaps state when pressed

  What a stuck value means

  No change

• Steering angle / seat and level sensors

  At rest / key on

  0.5 – 4.5 V through travel

  Working range

  Smooth sweep

  What a stuck value means

  Jumps or flat spots

Signal voltages assume a 5 V reference. Always compare against live data on the scan tool: a sensor reading right at the connector but wrong in data means a wiring or ECU problem.

State of charge from resting voltage (12 V lead-acid, 20 °C)

• 12.72 V or more

  State of charge

  100 %

  What to do

  Test as normal

• 12.60 V

  State of charge

  90 %

  What to do

  Fine for testing

• 12.45 V

  State of charge

  75 %

  What to do

  Charge before judging

• 12.30 V

  State of charge

  50 %

  What to do

  Recharge and retest

• 12.10 V

  State of charge

  25 %

  What to do

  Recharge, then test capacity

• 11.90 V or less

  State of charge

  Flat

  What to do

  Charge off the vehicle

• 10.5 V with one low cell

  State of charge

  Internal fault

  What to do

  Replace

Surface charge adds 0.1–0.3 V. Leave the car an hour, or switch the headlamps on for 30 seconds and wait a minute, before reading.

Cranking and starting

• Cranking voltage, petrol, 20 °C

  Pass

  Above 10.0 V

  Fail

  Below 9.6 V

  Notes

  Measured at the battery posts while cranking

• Cranking voltage, diesel, 20 °C

  Pass

  Above 9.6 V

  Fail

  Below 9.0 V

  Notes

  Higher compression, bigger draw

• Cranking voltage at -10 °C

  Pass

  Above 9.5 V

  Fail

  Below 9.0 V

  Notes

  Cold thickens the oil and slows the chemistry

• Voltage drop, battery positive to starter B+

  Pass

  Under 0.20 V

  Fail

  Above 0.50 V

  Notes

  While cranking, engine disabled

• Voltage drop, engine block to battery negative

  Pass

  Under 0.20 V

  Fail

  Above 0.30 V

  Notes

  While cranking

• Solenoid trigger voltage at the starter

  Pass

  Within 0.5 V of battery

  Fail

  More than 1 V low

  Notes

  Key held to crank

Charging system

• Charging voltage at idle

  Expected

  13.6 – 14.4 V

  Fail

  Under 13.2 V

  Notes

  Battery warm, no big loads

• Charging voltage at 2000 rpm, loaded

  Expected

  13.5 – 14.6 V

  Fail

  Under 13.0 V or over 15.0 V

  Notes

  Headlamps, blower and heated screen on

• Smart charging / variable voltage

  Expected

  12.4 – 15.0 V, varying

  Fail

  Fixed value with no variation

  Notes

  Common on stop-start and Euro 5 onward

• AC ripple at the battery

  Expected

  Under 0.05 V AC

  Fail

  Over 0.5 V AC

  Notes

  Engine running, meter on V~

• Voltage drop, alternator B+ to battery positive

  Expected

  Under 0.20 V

  Fail

  Above 0.40 V

  Notes

  Engine running, loaded

• Voltage drop, alternator case to battery negative

  Expected

  Under 0.10 V

  Fail

  Above 0.20 V

  Notes

  Engine running, loaded

The 3 % rule: total voltage drop across a circuit should not exceed 3 % of system voltage — about 0.4 V on a 12 V system, split between feed and earth.

Parasitic drain limits (vehicle fully asleep)

• Car built before 2000

  Acceptable

  Under 25 mA

  Investigate above

  35 mA

  Sleep time before reading

  10 – 20 minutes

• 2000 – 2010

  Acceptable

  Under 50 mA

  Investigate above

  80 mA

  Sleep time before reading

  20 – 45 minutes

• 2010 onward

  Acceptable

  Under 50 mA

  Investigate above

  80 mA

  Sleep time before reading

  45 – 90 minutes

• Telematics / connected car

  Acceptable

  Under 80 mA

  Investigate above

  120 mA

  Sleep time before reading

  Up to 2 hours, may wake periodically

• Drain that flattens a good battery overnight

  Acceptable

  Roughly 300 mA+

  Investigate above

  —

  Sleep time before reading

  60 Ah battery, 12 hours

Never open the door to reach the bonnet release once the test is set up — every wake-up restarts the sleep clock.

Typical current draw by load

• Starter, petrol 1.0 – 2.0 L

  Typical current

  120 – 200 A

  Fuse usually fitted

  Unfused main cable

  If it reads high

  Tight engine or failing starter

• Starter, diesel 2.0 L+

  Typical current

  250 – 400 A

  Fuse usually fitted

  Unfused main cable

  If it reads high

  Over 500 A

• Glow plugs (4 cylinder, initial)

  Typical current

  40 – 60 A total

  Fuse usually fitted

  60 – 80 A

  If it reads high

  One plug shorted

• Fuel pump (in-tank, petrol)

  Typical current

  4 – 8 A

  Fuse usually fitted

  15 – 20 A

  If it reads high

  Above 10 A

• Radiator fan, low speed

  Typical current

  8 – 15 A

  Fuse usually fitted

  30 A

  If it reads high

  Above 25 A

• Radiator fan, high speed

  Typical current

  20 – 35 A

  Fuse usually fitted

  40 – 50 A

  If it reads high

  Above 45 A

• Blower motor, speed 1 / speed 4

  Typical current

  4 A / 15 – 25 A

  Fuse usually fitted

  30 – 40 A

  If it reads high

  Above 30 A

• Headlamp, halogen H7 (each)

  Typical current

  4.6 A at 55 W

  Fuse usually fitted

  10 – 15 A per side

  If it reads high

  Above 6 A

• Headlamp, LED module (each)

  Typical current

  1 – 3 A

  Fuse usually fitted

  10 A

  If it reads high

  Pulsing draw

• Heated rear screen

  Typical current

  10 – 20 A

  Fuse usually fitted

  25 – 30 A

  If it reads high

  Under 5 A

• Heated seat element

  Typical current

  3 – 6 A

  Fuse usually fitted

  15 A

  If it reads high

  Intermittent

• Injector, saturated (holding)

  Typical current

  0.8 – 1.2 A

  Fuse usually fitted

  —

  If it reads high

  Above 1.5 A

• Injector, peak and hold

  Typical current

  4 – 6 A peak, 1 A hold

  Fuse usually fitted

  —

  If it reads high

  No peak

• Ignition coil primary (dwell)

  Typical current

  6 – 9 A

  Fuse usually fitted

  —

  If it reads high

  Above 12 A

• Central locking motor (each)

  Typical current

  2 – 5 A

  Fuse usually fitted

  20 – 30 A total

  If it reads high

  Above 8 A

• Electric window motor

  Typical current

  4 – 8 A running, 15 – 20 A stall

  Fuse usually fitted

  25 – 30 A

  If it reads high

  Stall current continuous

Clamp meters have a resolution floor. For milliamp drain testing use a low-current clamp or put the meter in series on the 10 A range with a bypass lead.

CAN, LIN and friends

• CAN High, idle

  Expected

  2.5 V, rising to 3.5 V

  Fault reading

  0 V or 5 V fixed

  What it means

  Measured to battery negative

• CAN Low, idle

  Expected

  2.5 V, falling to 1.5 V

  Fault reading

  0 V or 5 V fixed

  What it means

  Mirror image of High

• CAN termination (ignition off, battery disconnected)

  Expected

  60 Ω across High and Low

  Fault reading

  120 Ω or 40 Ω

  What it means

  Two 120 Ω resistors in parallel

• CAN line shorted to earth

  Expected

  —

  Fault reading

  Under 1 V on one line

  What it means

  Total bus failure or limp

• Single-wire CAN (body / comfort)

  Expected

  0 – 4 V switching

  Fault reading

  Flat 0 V or 12 V

  What it means

  Slower, fault-tolerant

• LIN bus

  Expected

  0 V to battery voltage switching

  Fault reading

  Flat

  What it means

  Single wire, master and slaves

• K-line (older diagnostics)

  Expected

  Around 12 V idle, pulled low to talk

  Fault reading

  0 V

  What it means

  Pin 7 on the OBD socket

• FlexRay

  Expected

  Differential 2.5 V pair, terminated

  Fault reading

  —

  What it means

  Chassis and dynamics systems

Measure termination resistance with the battery disconnected. Measuring a live bus with an ohmmeter gives nonsense and can upset communication.

Duty cycle and frequency you will actually meet

• Electronic throttle motor

  Typical figure

  10 – 90 % duty, 1 – 2 kHz

  Meter setting

  Duty % / Hz

  Notes

  Direction reverses with polarity

• Purge / EVAP valve

  Typical figure

  0 % at idle, up to 100 % cruising

  Meter setting

  Duty %

  Notes

  Audible ticking when active

• Radiator fan (PWM controlled)

  Typical figure

  10 – 100 %, 100 – 200 Hz

  Meter setting

  Duty %

  Notes

  Never fully off on some cars

• Alternator via LIN / RVC

  Typical figure

  Variable, commanded by ECU

  Meter setting

  Scan data

  Notes

  Not a fixed 14.4 V

• Wheel speed (active sensor)

  Typical figure

  7 mA / 14 mA square, 1 Hz per few km/h

  Meter setting

  Hz or scope

  Notes

  Current modulated

• MAF frequency type

  Typical figure

  2 kHz idle, 7 kHz full load

  Meter setting

  Hz

  Notes

  Digital output

• Fuel pump control module

  Typical figure

  Duty rises with demand

  Meter setting

  Duty %

  Notes

  Returnless systems

Ohm's law and power

• Volts

  Formula

  V = I × R

  Worked example

  2 A × 6 Ω = 12 V

  Use it for

  Predicting a drop across a known resistance

• Current

  Formula

  I = V ÷ R

  Worked example

  12 V ÷ 6 Ω = 2 A

  Use it for

  Checking a load draws what it should

• Resistance

  Formula

  R = V ÷ I

  Worked example

  12 V ÷ 2 A = 6 Ω

  Use it for

  Working out a load from its draw

• Power

  Formula

  P = V × I

  Worked example

  12 V × 4.6 A = 55 W

  Use it for

  Bulbs, heaters, fuse sizing

• Voltage drop from current

  Formula

  V = I × R

  Worked example

  100 A × 0.001 Ω = 0.1 V

  Use it for

  Starter cables

• Resistors in series

  Formula

  R total = R1 + R2

  Worked example

  2 Ω + 4 Ω = 6 Ω

  Use it for

  Corroded joints add up

• Resistors in parallel (two equal)

  Formula

  R ÷ 2

  Worked example

  120 Ω ∥ 120 Ω = 60 Ω

  Use it for

  CAN termination

Units and conversions

• 1 V

  Equals

  1000 mV

  Seen on

  Voltage drop tests

  Watch out for

  0.250 V and 250 mV are the same number

• 1 A

  Equals

  1000 mA

  Seen on

  Parasitic drain

  Watch out for

  0.045 A is 45 mA — a pass, not a fault

• 1 kΩ

  Equals

  1000 Ω

  Seen on

  Sensor resistance

  Watch out for

  2.5 kΩ is 2500 Ω

• 1 MΩ

  Equals

  1,000,000 Ω

  Seen on

  Insulation testing

  Watch out for

  0.5 MΩ can still be a leak

• 1 µF

  Equals

  1000 nF

  Seen on

  Capacitance

  Watch out for

  Piezo injectors

• °C to °F

  Equals

  (°C × 9 ÷ 5) + 32

  Seen on

  Temp sensor data

  Watch out for

  80 °C = 176 °F

Cable size, safe current and fuse colours

• 0.5 mm²

  Approx AWG

  20 AWG

  Safe continuous current

  Up to 11 A

  Typical use

  Sensor and signal wiring

• 1.0 mm²

  Approx AWG

  18 AWG

  Safe continuous current

  Up to 16 A

  Typical use

  Lighting, small motors

• 1.5 mm²

  Approx AWG

  16 AWG

  Safe continuous current

  Up to 21 A

  Typical use

  Blower feeds, heated seats

• 2.5 mm²

  Approx AWG

  14 AWG

  Safe continuous current

  Up to 27 A

  Typical use

  Fan feeds, heated screen

• 4.0 mm²

  Approx AWG

  12 AWG

  Safe continuous current

  Up to 36 A

  Typical use

  High-current accessories

• 6.0 mm²

  Approx AWG

  10 AWG

  Safe continuous current

  Up to 46 A

  Typical use

  Winch and inverter feeds

• 25 mm² +

  Approx AWG

  4 AWG and up

  Safe continuous current

  170 A +

  Typical use

  Starter and main earth

Standard blade fuse colours: 5 A tan, 7.5 A brown, 10 A red, 15 A blue, 20 A yellow, 25 A clear/white, 30 A green, 40 A orange.

Sanity rules worth memorising

• The 0.1 V earth rule

  The rule

  Any earth path should drop under 0.1 V under load

  Why

  Earths are the most common fault and the least tested

• The 3 % rule

  The rule

  Whole-circuit drop under 3 % of supply — about 0.4 V

  Why

  Anything more and the load underperforms

• Voltage without load proves nothing

  The rule

  Always load the circuit before you trust 12 V

  Why

  High-resistance joints pass voltage but not current

• Two probes, two points

  The rule

  Every voltage is a difference

  Why

  '12 volts at the connector' is meaningless alone

• Compare, do not guess

  The rule

  Read the same point on a known good bank, corner or cylinder

  Why

  Relative readings beat remembered specs

• Fix the earth first

  The rule

  Before replacing any module or sensor

  Why

  Bad earths generate implausible-signal codes everywhere