How to test · Actuator

How to test a petrol fuel injector: resistance, voltage waveform and balance

This page covers port fuel injectors on petrol engines. A faulty injector causes a misfire on one cylinder, a lean or rich code, rough idle or hard starting. Electrical tests prove the coil and the ECU control; a balance or swap test proves the fuel delivery. Direct-injection petrol injectors and diesel injectors run at much higher voltages and pressures and need different tests.

Updated 25 Oct 2026 · written by ipacen; typical values, not manufacturer data

In short

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. On a scope the control side shows battery voltage, a drop to near 0 V while the injector is open, then an inductive spike of roughly 35–100 V when it closes, often with a small bump where the needle seats.

Typical values

Check	Typical good	Suspect
Resistance, high-impedance	About 11–18 Ω, all within about 1 Ω of each other	Open, much lower, or one different
Resistance, low-impedance	About 2–5 Ω	Open or shorted
Supply with ignition on	Battery voltage	Missing: fuse, relay or wiring
Inductive spike when closing	Roughly 35–100 V, similar on all cylinders	Much lower: shorted turns
Pulse width at warm idle	Often about 2–4 ms	Much longer, with positive fuel trims: lean condition

Typical ranges, not manufacturer data: check the specification for the engine.

Wiring: what you should measure

Port injector (2-pin)

Wire	What you should see
Supply	Battery voltage with the ignition on (usually a common feed for all injectors)
Control	Switched to ground by the ECU; this is the side to scope

Before you start

Read misfire counters per cylinder and fuel trims; check fuel pressure if all cylinders look lean.
Rule out ignition and compression on the misfiring cylinder.

Multimeter and noid light

Unplug each injector and measure its resistance; compare all cylinders.
Ignition on: check battery voltage on the supply pin.
Plug a noid light into the connector and crank: it should flash on every cylinder.

Oscilloscope

Channel A on the control side, about 20 V/div and 1–2 ms/div at idle.
Check the "on" voltage drops to near 0 V, the spike is similar on every cylinder, and the needle-closing bump is visible.
With a low-amp current clamp, the current should ramp smoothly with a small kink where the needle opens.

Fuel delivery

Balance test: pressurise the rail, pulse each injector the same time and compare the pressure drop.
Swap test: move the suspect injector to another cylinder; if the misfire moves with it, the injector is faulty.

What it looks like on a scope

Good10 ms

Control side of a port injector: battery voltage, about 0 V while open, an inductive spike of about 70 V on closing and a small bump where the needle seats.

Fault: Shorted turns10 ms

A low spike and no needle bump: shorted turns in the coil or a sticking needle. Compare with the other cylinders.

Typical shapes drawn by ipacen, not a capture from one car: amplitude and timing vary with the engine, the sensor and the rpm.

What the readings tell you

You find	It usually means
No pulse on the control side	ECU driver, wiring, or no crank/cam signal (no sync)
Low spike and no needle bump	Shorted turns or a sticking needle
Electrically good, but one cylinder lean	Clogged injector (flow or balance test) or an intake leak at that runner
Misfire follows the injector after a swap	Faulty injector

Known engine problems behind these codes

1.9 TDI PD: Injector wiring loom under the cam coverVolkswagen, Audi, Skoda, Seat, Ford · typically 150,000–300,000 km 1.2/1.4 TSI EA111: Cracked pistons on the high-output 1.4 TSIVolkswagen, Audi, Skoda, Seat · typically 80,000–180,000 km 1.8/2.0 TFSI EA888: High oil consumption (piston rings)Audi, Volkswagen, Skoda, Seat · typically 60,000–160,000 km 1.8/2.0 TFSI EA888: Carbon build-up on the intake valvesAudi, Volkswagen, Skoda, Seat · typically 80,000–160,000 km 1.2 PureTech: High oil consumptionPeugeot, Citroën, DS, Opel · typically 60,000–160,000 km 1.6 THP/Prince: Oil consumption and carbon build-upMini, BMW, Peugeot, Citroën, DS · typically 60,000–160,000 km 1.2 TCe: High oil consumptionRenault, Dacia, Nissan, Mercedes-Benz · typically 40,000–120,000 km Theta II 2.0/2.4: Carbon build-up on the intake valves (GDI)Hyundai, Kia · typically 80,000–180,000 km

Questions mechanics ask

What resistance should a fuel injector have?

Most modern petrol port injectors are high-impedance, about 11–18 Ω; older low-impedance types are about 2–5 Ω. More important than the exact value is that all injectors on the engine read within about 1 Ω of each other.

How do I know if a fuel injector is clogged?

A clogged injector is usually fine electrically but delivers less fuel: one cylinder runs lean and misfires. A balance test (pressure drop per injector), a flow test or a swap test confirms it.

Can I test direct-injection injectors the same way?

No. Petrol direct injectors and diesel injectors are driven at much higher voltages and the fuel is at very high pressure. Use the scan tool tests, a current clamp and the manufacturer procedure, and never apply battery voltage to them directly.

Values are typical, not manufacturer data. Always follow the service information for the vehicle and work safely.