P28BC — Cylinder 12 Ion Current Sense Circuit

Causes

• Open or shorted ion‑sense wiring to cylinder 12
• Corroded, loose, or damaged connector at coil/spark plug/PCM
• Faulty coil‑on‑plug or ignition coil driver for cylinder 12
• Bad or fouled spark plug, wrong plug type, or excessive gap
• High resistance or poor ground in ignition/ion sensor circuit
• Water intrusion, carbon tracking or insulation breakdown around plug/coil

Symptoms

• MIL (Check Engine) illuminated with P28BC stored
• Possible single‑cylinder misfire detection or misfire counter increment for cylinder 12
• Rough idle or reduced smoothness depending on misfire severity
• Reduced fuel economy and increased emissions if misfire is occurring
• Intermittent faults that may clear when conditions change

What to check

• Pull freeze frame / live data and confirm MIL and when code set (idle, load, cold start).
• Visually inspect coil 12, spark plug, and harness for damage, corrosion, oil or water. Check connector locking tab and pins.
• Check spark plug condition and correct gap; remove plug to inspect electrode, ceramic for cracks or contamination. Replace if suspect.
• Swap coil 12 with a known good coil from another cylinder — see if the code or symptom follows the coil (use caution; clear codes and re-test).
• Backprobe the ion sense and coil supply/ground circuits at the connector with ignition off to check for continuity and shorts to ground/power (compare to a good cylinder).
• Check coil primary and secondary resistance against specification (compare to another known-good coil). Typical ranges (vehicle specific): primary ~0.3–3 Ω, secondary ~5 kΩ–20 kΩ — consult OEM.

Signal parameters

• Ion current waveform: a pulsed waveform synchronized to the ignition event with a characteristic post‑spark ionization spike. Absent, inverted, noisy or low amplitude indicates fault.
• Typical ion‑sense signal amplitude: vehicle and circuit dependent; expected measurable spike above baseline during combustion (often millivolt to volt range depending on internal circuitry). Use OEM oscilloscope reference waveforms.
• Frequency: matches firing events for the cylinder (once per spark event for that cylinder).
• Coil primary resistance (typical reference): ~0.3–3 Ω (high‑current coils vary by manufacturer).
• Coil secondary resistance (typical reference): ~5 kΩ–20 kΩ (varies widely by coil design).
• Spark plug gap reference: typically 0.6–1.2 mm depending on engine — use OEM spec.

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