Is P0270 worse than an open-circuit code?

It can be, because a hard short to earth stresses the ECU driver, whereas an open circuit simply stops the injector firing. That makes finding the short quickly more important.

Why is cylinder 4 prone to wiring shorts?

Its loom usually runs along the back of the engine near the exhaust and gearbox, where heat and movement wear the insulation faster than on the more sheltered front cylinders.

Can I test for the short without removing parts?

Often yes. With the injector unplugged you can measure from the control pins to earth and inspect the visible loom, which finds many shorts without major dismantling.

Will P0270 clear on its own?

No. The light may go out after the fault is repaired and the code cleared, but the short itself must be physically fixed first or it will keep returning.

Does this affect fuel economy?

Yes. A misfiring cylinder 4 makes the engine work harder and the ECU richen the mixture, so you will usually notice worse economy until it is repaired.

Should a garage check the ECU first?

No. Good practice is to rule out the cheap, common causes - wiring and the injector - before testing the ECU, which is the rarest and most expensive cause of P0270.

P0270 - Cylinder 4 Injector Circuit Low

What Is P0270?

P0270 is logged when the ECU finds the cylinder 4 injector control circuit shorted low or pulled toward earth. Instead of the clean switching pulse it expects, the ECU sees the voltage drop away, records the fault, and often shuts the injector channel down to protect its internal driver.

Cylinder 4 typically lives at the gearbox end of a transverse engine, where the loom routes close to hot and moving parts. A short there upsets cylinder 4's fuelling, causes a misfire, and can stress the ECU, so prompt diagnosis is worthwhile.

Common Symptoms

Misfire isolated to cylinder 4
Engine management light, possibly flashing
Rough idle and lost power
Injector fuse failures in some cases
Fuel smell if the injector sticks open
Higher fuel consumption

Common Causes

Short to earth - a chafed cylinder 4 control wire grounding out, the most likely cause given its routing.

Heat-damaged insulation - exhaust heat at the gearbox end breaking down the wire covering.

Internal injector short - the solenoid shorted within the injector, pulling the line low.

Wet or corroded connector - moisture bridging the connector to earth.

Crushed loom - the harness trapped against the bellhousing or a bracket.

ECU driver short - a failed low-side driver in the ECU, uncommon.

How to Diagnose P0270

Check Circuit State on the Scanner

With the ignition on, read the cylinder 4 injector circuit status. A short-low fault generally flags straight away, helping you separate it from a plain misfire.

Inspect the Hot Side of the Engine

Follow the cylinder 4 wiring along the back of the engine toward the gearbox. Heat damage and chafing are common here, and either can short the circuit to earth.

Pin-to-Earth Test

Unplug the injector and measure resistance from each control pin to earth. A low value confirms a short in the harness or connector.

Resistance Check the Injector

Compare cylinder 4's injector resistance with the others. A markedly lower reading means an internal short and a replacement injector.

Service the Connector

Inspect for corrosion and moisture, clean thoroughly, and reseal. Connectors at the rear of the engine collect road spray and are a frequent source of low-circuit faults.

Decide Wiring vs ECU

If the short to earth remains with the injector unplugged, the wiring is at fault. Repair it; only look at the ECU once the loom and injector are proven sound.

Avoid ECU DamageLeaving a short to earth active can overload the ECU's injector driver. Keep cranking and driving to a minimum until the short is located, or a simple wiring repair may turn into an ECU replacement.

Verdict

P0270 points to a short, so begin at the wiring along the hot gearbox end of the engine and test for a short to earth. Check the injector for an internal short next, and treat the ECU as the last suspect once the rest is clear.

Want the full picture? The OBD Fault Code Plain English Guide (PDF) covers the most common UK fault codes in one plain-English download.