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DTC codes / Volkswagen codes / P1240

P1240 · VolkswagenCylinder 4 Injector Circuit Open Circuit

P1240 on Volkswagen vehicles indicates that the engine control module (ECM) has detected an open circuit in the electrical path for the cylinder 4 fuel injector, meaning the expected current flow is absent when the injector is commanded on. This typically results in a misfire on cylinder 4 and may cause the engine to run rough or enter limp mode.

SystemPowertrain
Code typeVolkswagen specific
SeverityMedium
Fix difficultyIntermediate
AreaFuel and air metering (injector circuits)

Symptoms you may notice

  • Rough idle or engine misfire, especially noticeable at idle or low speeds
  • Check Engine Light illuminated, often flashing if the misfire is severe
  • Reduced engine power or limp mode activation
  • Shaking or vibration from the engine bay
  • Often no symptoms besides the warning light if the injector circuit is intermittently open

Common causes, most likely first

  1. Faulty or damaged fuel injector for cylinder 4 (open coil winding)
  2. Corroded, loose, or damaged electrical connector at the cylinder 4 injector
  3. Broken or chafed wire in the injector harness, often near the cylinder head or intake manifold
  4. Open circuit in the injector driver wire between the ECM and the injector
  5. Failed ECM injector driver (less common but possible)
  6. Rodent damage to the wiring harness

How to diagnose and fix it

  1. Perform a visual inspection of the cylinder 4 injector connector and wiring for damage or corrosion
  2. Disconnect the injector and measure resistance across its terminals; compare to the other injectors (typically 12-16 ohms)
  3. Check for battery voltage at the injector connector with the key on, engine off, using a multimeter or test light
  4. Inspect the harness for continuity between the ECM and injector, and check for shorts to ground or power
  5. Swap the cylinder 4 injector with another cylinder to see if the misfire moves, confirming injector fault
  6. If the injector tests good, check the ECM driver signal with an oscilloscope or noid light
  7. Replace the faulty injector or repair the wiring as needed, then clear codes and verify the repair

Common Misdiagnosis Traps for P1240

  • A misfire on cylinder 4 does not always mean the injector itself is bad; the open circuit could be in the wiring or connector, so always test the circuit before replacing parts.
  • Volkswagen engines often suffer from brittle wiring insulation near the cylinder head due to heat cycling, leading to intermittent open circuits that only appear when the engine is hot or vibrating.
  • The ECM may set P1240 without storing a misfire code if the open circuit is intermittent, so do not rely solely on misfire counters to confirm the fault.
  • Aftermarket injectors with incorrect resistance values can trigger this code even when new, so always verify resistance against the factory specification.
  • Corrosion inside the injector connector can cause an open circuit that is not visible from the outside; disconnecting and inspecting the pins is essential.

Can I keep driving with P1240?

Driving with a confirmed cylinder 4 injector open circuit is not recommended because the misfire can damage the catalytic converter and cause poor engine performance. If the check engine light is flashing, stop driving immediately and have the vehicle towed to a repair shop.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

Guidance is general and written for Volkswagen vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

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