P28A1

Code

Generic P — Powertrain

Cylinder 5 Ion Current Sense Circuit Range/Performance

Brand: Generic

Type: P — Powertrain

Views: UK: 18 EN: 31 RU: 21

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Page language: EN

Causes

Open, short or high-resistance wiring in the ion-sense circuit for cylinder 5
Poor connector connection, corrosion or damaged pins at the coil/ECM
Faulty ignition coil or coil pack for cylinder 5 (ion-sensing coil)
ECM/PCM internal fault or incorrect software/calibration
Severe fuel/air mixture or ignition problems in cylinder 5 (weak spark, fouled plug)
Mechanical engine issues (low compression, blown head gasket) affecting ionization

Symptoms

Malfunction indicator lamp (MIL) / Check Engine Light illuminated
Intermittent or continuous misfire on cylinder 5 (may store P0305)
Rough idle or reduced engine performance
Poor fuel economy
Possible difficulty starting or uneven running under load
Diagnosis trouble codes related to ignition or misfire may be present

What to check

Read freeze frame and all stored/related codes; note operating conditions when code set
Visually inspect coil, spark plug, wiring harness and connector for cylinder 5 for damage, corrosion or oil contamination
Check coil power and ground circuits and connector retention
Swap coil (or ion-sensing coil) from cylinder 5 with another cylinder and see if the code or symptom follows the coil
Scan live data for ion current or ion-sense waveform (if available) and compare cylinder 5 to a known-good cylinder
Check for related misfire codes (e.g., P0305) and perform a compression/leak-down test on cylinder 5

Signal parameters

Ion-sense signal: short post-spark ionization pulse measurable at coil/ECM input; should be consistently present each firing event
Amplitude: must be clearly above background/noise level and comparable to other cylinders (exact values model-dependent)
Timing: pulse appears within a short window after the spark event and aligns with cylinder 5 timing
Shape: expected pulse waveform shape (presence and decay) should match other cylinders; excessive noise, missing peak or abnormal decay indicates fault
Diagnostic comparison: relative magnitude, timing and repeatability vs a known-good cylinder are key—absolute voltages vary by manufacturer

Diagnostic algorithm

Safety and preparation: park on level ground, apply parking brake, disconnect battery only if required by repair procedures; follow vehicle-specific safety procedures.
Read codes and freeze frame with a capable scan tool. Note engine speed, load, temperature when P28A1 set. Retrieve any related codes (misfire, ignition, sensor).
Perform a visual inspection of coil(s), spark plug(s), wiring harness and connectors for cylinder 5. Repair obvious damage or corrosion.
If coils are user-serviceable and interchangeable, swap the coil (and plug if needed) from cylinder 5 with another cylinder. Clear codes and test-drive or crank to see if the code follows the coil (if it does, likely coil).
Using a scan tool or oscilloscope, capture the ion-sense waveform for cylinder 5 and compare to a nearby cylinder during the same operating conditions. Look for missing pulse, low amplitude, excessive noise or timing shift.
Verify coil primary/secondary power and ground, and check connector pin continuity between coil and ECM. Repair any open/short/high-resistance circuits.

Likely causes

Failed ion-sensing ignition coil on cylinder 5
Wiring harness damage or connector corrosion at cylinder 5 coil
Loose or poor ground or power supply to the coil/ion circuit
Intermittent connector contact or pin damage at the ECM
Cylinder 5 combustion anomaly (misfire, low compression, leaking injector) producing atypical ion signature

Fault status

⚠️ Status

ECM detected ion current sense signal for cylinder 5 outside expected range or with abnormal waveform/performance. Monitoring of combustion on cylinder 5 is unreliable. MIL illuminated and fault stored.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 1.0-3.0 hours

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