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P035CIgnition Coil "O" Primary/Secondary Circuit

The PCM has detected a fault in the primary or secondary circuit of ignition coil "O", which fires cylinder 15 on engines with fifteen or more cylinders.

Definition

The PCM commands each ignition coil primary circuit and verifies the resulting feedback voltage to confirm the coil actually fired. P035C sets when the coil "O" (cylinder 15) circuit is open, shorted, or produces no feedback, meaning spark to that cylinder is unreliable or absent. The result is a cylinder 15 misfire with rough running, power loss, and MIL illumination — a flashing MIL indicates catalyst-damaging misfire. This designator exists only on engines with fifteen or more cylinders, such as W16 applications.

Common causes

  • Failed ignition coil for cylinder 15
  • Damaged or corroded coil electrical connector
  • Open, chafed, or shorted coil circuit wiring
  • Missing voltage supply to the coil (fuse or relay)
  • Bad ground path for the coil bank
  • Faulty PCM output driver (least common)

Common misdiagnoses

  • Spark plugs replaced when the fault is electrical, not secondary wear
  • Injector or fuel-system parts replaced for an ignition-side misfire
  • PCM condemned before circuit testing
  • Entire coil bank replaced for a single-coil circuit fault

Troubleshooting steps

  1. 1.Perform a coil swap test

    Exchange the cylinder 15 coil with a neighbor and recheck; a code that follows the coil confirms a bad coil.

  2. 2.Inspect the connector and wiring at the coil

    Look for terminal corrosion, loose lock tabs, oil intrusion, and insulation chafe on the sub-harness.

  3. 3.Verify supply voltage and ground

    Confirm battery voltage on the feed circuit key-on and a good ground with a voltage-drop test (under 0.1 V).

  4. 4.Check coil primary resistance

    An open or out-of-spec primary (spec is typically about 0.4-2.0 ohms) confirms coil failure.

  5. 5.Verify the PCM control signal with a scope

    Confirm switching on the control circuit while cranking; missing commands with good wiring continuity point to the PCM.

  6. 6.Repair and retest under load

    After repair, clear codes and run the engine under load to confirm no misfire counts accumulate on cylinder 15.

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