P28B9

Code

Generic P — Powertrain

Cylinder 11 Ion Current Sense Circuit Range/Performance

Brand: Generic

Type: P — Powertrain

Views: UK: 21 EN: 32 RU: 24

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Page language: EN

Causes

Open, short, or high-resistance wiring or poor connector at the ion-sense circuit for cylinder 11
Faulty ignition coil or coil-pack for cylinder 11 (internal damage or short)
Bad or fouled spark plug on cylinder 11 or incorrect plug gap
Faulty fuel injector or injector electrical problem on cylinder 11 causing weak/no combustion
Severely low compression, valve or mechanical problem on cylinder 11
Corroded/poor ground or power supply affecting the ion sensing circuit

Symptoms

Malfunction Indicator Lamp (MIL) illuminated with P28B9 stored
Cylinder 11 misfire, rough idle, or reduced power
Increased fuel consumption and emissions
Intermittent misfires or driveability problems concentrated to one cylinder
Related misfire DTCs may also be present

What to check

Retrieve freeze frame and live data for ion current and misfire counters; note conditions when fault set
Inspect wiring harness and connector for cylinder 11 ion sense (look for damage, corrosion, broken pins)
Backprobe or monitor the ion sense signal with a suitable diagnostic oscilloscope or scan-tool live-data
Inspect spark plug for wear, fouling, correct gap, and correct part number
Swap ignition coil between cylinder 11 and a known-good cylinder to see if code follows coil
Swap injector (or test injector operation) to isolate fuel delivery problems

Signal parameters

Ion current waveform present shortly after ignition event; waveform amplitude and shape vary with combustion quality
Typical ion-current magnitude is very small (microamp-level) and appears as a short pulse coincident with combustion
No signal (open) or a constant/flat signal indicates loss of sensing; excessive noise or erratic pulses indicate wiring/EMI or component faults
Timing of the ion current pulse should align with ignition timing for cylinder 11

Diagnostic algorithm

Verify DTC: Clear codes and attempt to re-run the fault to reproduce. Record freeze-frame and environmental conditions.
Scan for related codes: Note misfire codes and any crank/cam sensor faults that could affect timing or sensing.
Visual inspection: With ignition off, inspect wiring harness, connectors, and shielding for cylinder 11 ion-sense circuit. Repair any physical damage.
Spark plug/coil check: Remove and inspect spark plug for fouling or damage; measure gap. Swap the ignition coil with a known-good cylinder and re-test. If code follows coil, replace coil.
Injector check: Verify injector electrical supply and pulse. Swap injector with another cylinder or test flow to rule out fuel delivery problems.
Electrical tests: Backprobe the ion-sense circuit and check for correct reference voltages and ground. Measure continuity and resistance to ECM. Check for shorts to battery or ground.
Oscilloscope check: Use a scope to view the ion-current waveform during cranking and at idle/run. Compare waveform shape, amplitude and timing to a known-good cylinder.
Mechanical check: If electrical and ignition items check good, perform a compression or leak-down test on cylinder 11 to rule out mechanical causes of poor combustion.
ECM/Software: If wiring, ignition, fuel and mechanical systems are good and waveform is absent or abnormal only at the ECM input, consider ECM/ignition module failure or corrupted software. Confirm with manufacturer bulletins and reflash or replace ECM only after excluding other causes.
Repair and verify: Repair identified faults, clear codes, and perform road or drive-cycle verification to ensure the ion-sense waveform and function are restored and code does not return.

Likely causes

Wiring/connectors to ion-sense circuit (intermittent or high resistance)
Faulty ignition coil or spark plug on cylinder 11
Injector not delivering fuel (electrical or mechanical)
Poor ground at ignition/ECM
ECM or ignition driver failure (less common)

Fault status

⚠️ Status

Cylinder 11 ion-current sensing is outside expected range or performance. Possible wiring/ignition/fuel or combustion problem on cylinder 11, or a sensor/ECM circuit fault. Inspect wiring, coil/plug, injector, and verify ion-current waveform before replacing control module.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 1.5-4.0 hours