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Injector driver or injector? How to tell before you send the ECU away

One cylinder is dead, there's an injector circuit fault stored, and the question is the same every time: is it the injector, the wiring, or the injector driver inside the ECU? Get it wrong and you either replace a good ECU or fit a repaired one that fails again within a week. A few fundamentals settle it before anything leaves the workshop.

How the circuit works

On most port-injected petrol engines the injector is a simple solenoid. One pin is fed switched battery voltage, usually through a fuse or relay shared with the other injectors. The other pin goes back to the ECU, where a transistor — the driver — switches it to earth to open the injector. Each injector normally has its own driver, so one dead driver means one dead cylinder.

That gives you three places the fault can be: the injector coil, the wiring between the injector and the ECU, or the driver itself.

Symptoms

• A misfire on one cylinder that doesn't move when you swap the plug or the coil
• An injector circuit code (open or short) rather than only a misfire code
• Sometimes a hard start or rough idle if the injector is stuck open rather than dead

How to check it

1. Supply first. With the ignition on and the injector unplugged, one pin on the harness side should show battery voltage. No supply on one injector only points to wiring; no supply on all of them points to the fuse or relay, not the ECU.
2. Measure the injector coil. Compare its resistance with the injectors on the other cylinders and with the specification for that engine. Many high-impedance port injectors read somewhere around 12–16 Ω, low-impedance types only a few ohms — check the data for the engine you're on. An open coil, or one reading far below its neighbours, is a faulty injector.
3. Swap it. Move the suspect injector to a neighbouring cylinder. If the misfire follows the injector, the injector is the fault. If it stays on the same cylinder, look at the wiring and the driver.
4. Check the signal wire. Check continuity from the injector connector to the ECU pin, and check that wire for a short to earth and a short to battery voltage. A wire chafed against the block is a common cause.
5. Look for the switching signal. With the injector connected, a scope on the ECU side of the injector shows whether the driver is switching it. Compare the dead cylinder with a good one. If supply, coil and wiring are all good and that one channel never switches while the others do, the driver is the likely fault.

Why the order matters

A driver rarely fails on its own. The usual cause is an injector with a shorted coil, or a signal wire shorted to battery voltage, pulling more current through the driver than it was built for until it burns out. Repair the ECU without fixing that and the new driver fails the same way. So when a driver has failed, find out what killed it, and fix that before the repaired unit goes back in.

A word on direct injection and diesels

Direct-injection petrol engines and common-rail diesels don't work like the circuit above. Their injectors are driven with a high-voltage opening pulse, often well over 50 volts, before dropping to a holding current. A test light or the checks above can damage the circuit or give you a shock. Use the correct procedure and specification for that system, and treat it as a specialist job if you're not sure.

When to call a specialist

If supply, injector and wiring all check out and the channel still doesn't switch, the ECU needs a bench test. That's where we come in: we test the unit, tell you what we find and what it costs before any work, and repair the driver where it's viable.