Fuel and air

P0285 — Cylinder 9 Injector Circuit Low

Severity: medium Drive with care, to a shop

What P0285 means

The engine control module fires the cylinder 9 injector by switching its circuit to ground with an output driver, and it monitors the voltage in that circuit. P0285 sets when the voltage stays lower than expected, for example from a wire shorted to ground, a faulty injector coil or a failing driver. The code only applies to engines with nine or more cylinders, such as V10 and V12 designs, or to makers who number cylinders in a way that includes cylinder 9.

Symptoms

  • Check Engine light on
  • Rough idle
  • Misfire on cylinder 9
  • Reduced power under load
  • Higher fuel consumption
  • Hard starting or stalling

Possible causes

  1. Faulty cylinder 9 injector (open or shorted coil)
  2. Open circuit or short to ground in the injector wiring harness
  3. Corroded, loose or damaged injector connector
  4. Poor ground connection in the injector circuit
  5. Failed injector driver inside the ECM
  6. Fuel pressure problems (unlikely cause)

How to diagnose

  1. Scan all codes and freeze frame data and look for other injector codes
  2. Inspect the cylinder 9 injector harness and connector for chafing, corrosion and oil contamination
  3. Measure injector coil resistance and compare it to the spec for your engine and to neighboring injectors
  4. Test the power and control wires for opens and shorts to ground
  5. Check ground connections and injector power supply
  6. Use a scope or noid light to confirm the ECM sends injector pulses
  7. Compare fuel trims and cylinder balance, and check fuel pressure
  8. If wiring and injector test fine, test the ECM driver

Common fixes

  • Replace the faulty cylinder 9 injector
  • Repair or replace the damaged wiring and connector
  • Restore a clean, tight ground connection
  • Repair or replace the ECM if the driver has failed
  • Clear the code and confirm it does not return

Questions and answers

What does P0285 mean?

The ECM sees low voltage in the cylinder 9 injector control circuit. The cause can be the injector itself, its wiring or connector, or the driver in the control module.

Can I drive with P0285?

A short trip to a shop is usually fine, but the engine may misfire and lose power. Driving for long risks damaging the catalytic converter and wasting fuel.

How do I tell a bad injector from a wiring problem?

Measure the injector coil resistance and check the harness for opens and shorts to ground. If the injector is within spec but no control pulses arrive, look at the wiring or the ECM.

This is general information about the code. The meaning can differ between makes and models: a scan tool and the repair manual for your car will show the exact cause.