P28AF

Code

Generic P — Powertrain

Cylinder 8 Ion Current Sense Circuit High

Brand: Generic

Type: P — Powertrain

Views: UK: 29 EN: 24 RU: 24

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Page language: EN

Causes

Open, shorted or corroded wiring/connectors in the cylinder 8 ion-current/ignition circuit
Faulty ignition coil or coil pack (secondary leakage, internal short) for cylinder 8
Damaged or incorrectly gapped spark plug on cylinder 8
Short to battery voltage or loss of reference/ground on the ion-sense circuit
Fuel or oil contamination on the spark plug (leading to altered ion current)
Severe detonation/abnormal combustion raising ion current level

Symptoms

MIL (Check Engine Light) illuminated
Misfire or rough running, especially on cylinder 8
Poor idle quality, reduced power or fuel economy
Occasional hard start or inconsistent ignition behavior
Related misfire codes (P0308) or ignition coil codes may also be present

What to check

Read freeze-frame data and all stored codes with scan tool; note any concurrent misfire (P0308) or ignition/injector codes
View live-data ion-current (or “ion” / post-spark voltage) for all cylinders; compare cylinder 8 to others
Visually inspect ignition coil, spark plug and wiring harness for cylinder 8 for damage, corrosion or oil/fuel contamination
Check connector pins for bent, pushed-out or corroded terminals at the coil and PCM
Check for water/oil intrusion into spark plug well if applicable (direct‑coil engines)
Perform a basic ignition secondary test (spark, coil resistance per manufacturer) and inspect spark plug condition

Signal parameters

Ion-sense signals vary by manufacturer; typical expected behavior: small-amplitude post-spark pulses near 0 V baseline. High/fault condition = steady elevated DC offset or pulse amplitude above manufacturer threshold.
Typical magnitude examples (vehicle-dependent): normal post-spark ion currents usually register as small mV–V level pulses; a high fault often appears as sustained voltage well above the other cylinders (often >0.5–1.0 V depending on system).
Timing: the ion-current pulse should occur immediately after the ignition event (after coil discharge); compare phase and amplitude to adjacent cylinders.

Diagnostic algorithm

Confirm code and record freeze-frame/live data. Do not assume final fault based on one code; duplicate the fault if possible.
Inspect coil-on-plug assembly and spark plug for cylinder 8. Replace or clean if fouled, shorted, or damaged. Re-test.
With ignition off, inspect wiring and connector at the coil and follow the ion‑sense wire back toward the PCM. Repair any damaged insulation, corrosion, or poor pin connection.
Use a DVOM to check for shorts: measure resistance from ion-sense pin to battery positive and to ground. Look for unexpected continuity to battery (short) or open circuit.
Swap the cylinder 8 coil and spark plug with a known-good cylinder (e.g., cylinder 7). Clear codes and run engine: if the code follows the coil/plug, suspect that component. If it stays on cylinder 8, focus on wiring or PCM input.
If available, use an oscilloscope to view the ion-current waveform relative to ignition primary/secondary. Compare cylinder 8 waveform to a known-good cylinder to identify abnormal DC offset, noise, or missing pulses.
Inspect spark plug well for oil/fuel; correct valve cover/seal or injector issues if contamination present.
Check fuel injector operation for cylinder 8 and perform a compression or leakdown test if suspecting abnormal combustion or mechanical problems.
If wiring and components test OK and the fault persists, consult manufacturer wiring diagrams and PCM pinouts. Check PCM grounds and reference voltages. Consider module replacement only after exhausting wiring and component checks.
After repair, clear codes and perform a road or extended idle test while monitoring ion-current live data to confirm proper readings and that the code does not return.

Likely causes

Faulty ignition coil or coil pack for cylinder 8
Damaged or fouled spark plug on cylinder 8
Wiring or connector fault (short to battery or poor ground) at cylinder 8 ion-sense circuit
Contamination (oil/fuel) on the plug or in the combustion chamber

Fault status

⚠️ Status

PCM reports cylinder 8 ion-current sense circuit voltage higher than expected. This indicates an abnormally high ion-current measurement or a circuit bias high condition for cylinder 8; possible causes include ignition coil or spark plug faults, wiring short to battery, contamination, abnormal combustion, or PCM input failure.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 1.5-4.0 hours