How to test sensors and actuators
Prove the part before you replace it. Each test gives the wiring, typical values, the steps with a multimeter, a scope and a scan tool, and what a good and a faulty waveform look like. Values are typical: always check the specification for the engine.
Sensors
Crankshaft position sensor (CKP)
Find out the type first. An inductive (2-wire) CKP sensor usually measures about 200–2,000 Ω and makes an AC signal that should reach at least about 0.5 V AC while cranking, rising with rpm.
How to test it →Camshaft position sensor (CMP)
Most camshaft sensors are Hall-effect: check for a 5 V or 12 V supply, a ground with less than about 0.1 V drop, and a signal that switches cleanly between about 0 V and 5 V as the engine turns. On a scope, compare it with the crankshaft signal: the cam pattern must line up with the same crank teeth every time, or the timing is out.
How to test it →Mass air flow sensor (MAF)
On a warm petrol engine at idle, a MAF sensor typically reads roughly 1 g/s per litre of engine size (about 2–3 g/s on a 2.0), and the reading should rise quickly and smoothly when you snap the throttle. An analogue sensor's signal is usually about 1 V at idle and 4–4.5 V at full load; a digital (frequency) sensor outputs a few kilohertz that rises with airflow.
How to test it →MAP and boost pressure sensor
With the key on and the engine off, a MAP sensor should read the same as barometric pressure, about 100 kPa at sea level: compare the two in live data. At warm idle a healthy naturally aspirated petrol engine pulls the reading down to roughly 25–40 kPa.
How to test it →Oxygen (lambda) sensor
A healthy narrowband (zirconia) oxygen sensor before the catalytic converter switches between about 0.1 V (lean) and 0.8–0.9 V (rich) several times a second at a steady 2,500 rpm with the engine warm. Each switch should be quick, well under about 300 ms.
How to test it →Coolant temperature sensor (ECT)
Most coolant temperature sensors are NTC thermistors: resistance falls as the engine warms, typically from about 2–3 kΩ at 20 °C to about 200–300 Ω at 90 °C. The quickest check is a cold engine that has stood overnight: coolant and intake air temperature in live data should agree within a few degrees.
How to test it →Diesel rail pressure sensor
A common-rail pressure sensor usually has three wires: 5 V reference, ground and a signal of about 0.5 V at zero pressure, rising towards about 4.5 V at maximum. With the engine off long enough for the rail to depressurise, live data should show close to 0 bar.
How to test it →Knock sensor
Most knock sensors are piezoelectric: they make a small AC voltage when the engine block vibrates, so a resistance check rarely proves much (many read very high or open). The useful test is a scope on the signal: tapping lightly on the block near the sensor should give clear spikes, often up to about 1 V.
How to test it →Throttle and accelerator pedal position sensors
Electronic throttles and accelerator pedals use two position signals for safety, each with a 5 V reference and a ground. With the ignition on, sweep slowly: track 1 typically rises from about 0.5–1 V at rest to about 4–4.5 V at full travel, and track 2 either mirrors it the other way or follows it at a different level (often about half).
How to test it →ABS wheel speed sensor
Find out the type. A passive (inductive) wheel speed sensor typically measures about 800–2,000 Ω and makes an AC voltage when the wheel turns, rising with speed.
How to test it →Actuators
Petrol fuel injector (port injection)
A typical high-impedance petrol port injector measures about 11–18 Ω (low-impedance types about 2–5 Ω), and all injectors on the engine should be within about 1 Ω of each other. With the ignition on, one pin has battery voltage and the ECU switches the other to ground.
How to test it →Ignition coil (coil-on-plug)
Most modern cars use coil-on-plug units with a built-in driver: check for a 12 V supply, a good ground and a trigger signal from the ECU that switches between about 0 V and 5 V. Coils without a driver can be measured: primary typically about 0.3–1 Ω, secondary about 6–15 kΩ.
How to test it →Diesel glow plugs
Disconnect the feed and measure each glow plug to ground: a working 12 V metal glow plug typically reads well under 2 Ω (often about 0.5–1.5 Ω), and an open circuit means it has failed. Compare all plugs: one far higher than the others is suspect.
How to test it →EGR valve
On most modern engines the EGR valve has a motor and a position sensor: run the actuator test with a scan tool and the actual position should follow the commanded position within a few percent, quickly in both directions. The position sensor usually has a 5 V reference, a ground and a signal of about 0.5–4.5 V.
How to test it →Turbo actuator (variable geometry)
Start with a road test: on a full-load pull in 3rd gear, actual boost should follow specified boost closely without overshooting. On a vacuum actuator, the rod should travel smoothly through its full stroke under a hand vacuum pump and hold vacuum, and the boost control solenoid should deliver vacuum in proportion to its duty cycle.
How to test it →Common-rail diesel injector
Injectors are judged by comparison. Run a leak-off (return) test at warm idle and compare cylinders: one returning more than about twice the average of the others is worn or leaking.
How to test it →