Code
P1305
HUMMER
P — Powertrain
Ignition Coil 2 Primary Feedback Circuit
AI status
Completed
Completed
100%
Causes
- Open or shorted wiring in coil 2 primary/feedback circuit
- Poor or corroded connector/terminal at ignition coil or PCM
- Failed ignition coil (internal short or open)
- Weak battery or poor chassis/engine ground
- Faulty PCM/ECM (rare) or incorrect PCM software
Symptoms
- Check Engine Light (MIL) illuminated
- Rough idle or engine misfire at cylinder 2
- Reduced power, hesitation, or poor acceleration
- Possible increased fuel consumption and visible misfire codes (P0302)
- Hard starting or stalling in severe cases
What to check
- Retrieve freeze frame and live data with a scan tool; note conditions when code set
- Check for additional codes (misfire or other coil circuits)
- Visually inspect coil 2 connector, wiring harness and PCM connector for damage or corrosion
- Measure battery voltage and main grounds before and during cranking
- Measure coil 2 primary resistance with ignition OFF and compare to spec
- Backprobe coil 2 primary/feedback pin while cranking to observe switching using a DVOM or oscilloscope
Signal parameters
- Primary coil resistance: typically low (example range 0.2–2.0 Ω depending on coil type) — check OEM spec
- Coil primary supply: near battery voltage (11–14.5 V) with ignition ON
- Primary driver signal: pulsed switching; voltage will transition between near-battery and near-ground during firing
- Feedback/current sense: PCM expects a repeatable current/voltage waveform during dwell and spark events
- No high-voltage testing while engine is running without proper insulated tools and precautions
Diagnostic algorithm
- Read and record all DTCs, freeze frame and live data. Note engine speed, load and battery voltage when fault occurred.
- Perform a visual inspection of coil 2, its connector, wiring harness and PCM connector for damage, corrosion, or loose pins.
- Check battery state and main engine/chassis grounds; repair any poor connections.
- With ignition OFF, disconnect coil 2 and measure primary winding resistance; compare to OEM specification. Replace coil if out of range or open.
- Reconnect, backprobe the coil primary/feedback terminal. With a DVOM or preferably an oscilloscope, crank/run engine and observe waveform. Look for missing pulses, constant open, or shorted signal.
- If waveform/signal is abnormal, inspect continuity between coil connector and PCM pin for opens/shorts to power or ground. Repair wiring as needed.
- Swap coil 2 with another cylinder's coil (if coils are identical) and clear codes. If code moves to the swapped cylinder, replace the coil. If it remains on cylinder 2, continue wiring/PCM checks.
- If wiring checks good and coil replacement does not clear the fault, suspect PCM driver failure and consult manufacturer service information before replacing PCM.
- After repairs, clear codes and test drive to confirm the fault does not return. Re-scan for stored codes and monitor live data.
Likely causes
- Damaged harness between coil 2 and PCM (abrasion, rodent damage, pinched)
- Oil/water contamination or corrosion on coil electrical connector
- Ignition coil 2 internal primary winding failure
- Intermittent ground or battery supply to ignition circuit
- PCM driver transistor failure
Fault status
Status
PCM detected an abnormal or missing primary feedback/current signal from Ignition Coil 2. This indicates a wiring, connector, coil or PCM driver problem; the PCM stored P1305 and may set a misfire code if combustion was affected.
Repair difficulty: Medium
Diagnostic time: 0.5-2.0 hours
Repair manuals
Manual library for HUMMER
138
Browse 138 HUMMER manuals: repair procedures, diagnostics, wiring diagrams, component locations, service data and Labor Times by year, model and trim.
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