Code
P1620
ALFA ROMEO
P — Powertrain
Gas ECU fault
AI status
Completed
Completed
100%
Causes
- Intermittent or lost communication between main ECU and gas (LPG/CNG) ECU (CAN or dedicated comm line)
- Low or missing power supply to the gas ECU (blown fuse, relay, ignition feed)
- Poor or high-resistance ground at the gas ECU
- Damaged/loose connector or corroded pins at the gas ECU or wiring harness
- Short or open in wiring between gas ECU and vehicle main ECU or instrument cluster
- Faulty gas ECU (internal processor, memory or circuitry)
Symptoms
- MIL/Check Engine lamp illuminated (may show gas ECU related message)
- Vehicle may refuse to switch to gas operation, default to petrol-only mode or enter limp mode
- Engine runs poorly on gas: rough idle, hesitation, misfire or stalling
- No communication with gas ECU when connected to diagnostic tool or intermittent diagnostics response
- Switching between petrol and gas may fail or be erratic
- Possible reduced engine power or drivability problems
What to check
- Read and record all stored and pending DTCs and freeze frame data from both main engine ECU and gas ECU
- Attempt to communicate with gas ECU using an appropriate diagnostic tool; note if ECU appears online
- Check fuses and relays related to the gas system and ECU (remove and visually inspect; test for continuity)
- Visually inspect gas ECU and harness for water ingress, corrosion, bent pins, damaged insulation, or recent mechanical damage
- With ignition ON (engine off) measure supply voltage at the gas ECU power input pins and continuity to battery/GND
- Check ground integrity: measure voltage drop between gas ECU ground and chassis battery negative during key-on cranking
Signal parameters
- Battery voltage at fuse/relay with ignition ON: approx. 12–14.5 V
- Ignition-switched feed to gas ECU: near battery voltage with key ON (within ~0.5–1.0 V of battery)
- Ground continuity: near 0 Ω to chassis/negative battery (voltage drop
- CAN bus idle voltages (typical): CAN_H ≈ 2.5–3.5 V, CAN_L ≈ 1.5–2.5 V; both near ~2.5 V if recessive. Dominant state: CAN_H ~3.5 V, CAN_L ~1.5 V
- Bus termination resistance: ~60 Ω across CAN_H and CAN_L (two 120 Ω in parallel)
- Typical solenoid/injector coil resistance (gas injectors): varies by design; commonly a few ohms to tens of ohms — consult manufacturer spec
Diagnostic algorithm
- Record all codes from engine and gas ECUs and capture freeze-frame/live data. Note if P1620 is current or intermittent.
- Attempt to communicate with the gas ECU using dealer or aftermarket scan tool that supports the gas system. If no comms, proceed to step 3.
- Verify fuses and relay(s) for the gas ECU. Replace any blown fuses and bench-test or swap suspected relays (with known-good unit) to confirm.
- With ignition ON, measure supply voltage at the gas ECU power pin(s). If low or absent, trace wiring back to fuse/relay and battery for opens or high resistance.
- Measure voltage drop on the ground(s) for the gas ECU with engine cranking and key ON. Repair any high-resistance grounds (clean, tighten, replace conductor).
- Inspect ECU connectors for corrosion, bent pins, water entry or mechanical damage. Clean contacts and reseat connectors; repair damaged pins or harness as required.
- Check communication lines: measure CAN_H and CAN_L idle voltages and check for approx. 60 Ω termination across them. Use oscilloscope if available to view bus activity and noise.
- Perform continuity/resistance checks between gas ECU and main ECU communication pins and between gas ECU and chassis ground/power. Repair damaged wiring or shorts.
- If communication is intermittent, perform wiggle/replication tests on harness with engine running while monitoring comms to locate intermittent wiring faults.
- If wiring, power and ground are verified good but the gas ECU does not communicate or reports internal errors, consult manufacturer service information for software updates, reflash procedures or module initialization steps.
- If software reflash/repair is unsuccessful and manufacturer guidance indicates, consider gas ECU replacement. After replacement, perform necessary coding/parameterization and full system test.
- Clear codes, perform a functional test of the gas system (switching, leak checks, road test) and confirm no recurrence of P1620.
Likely causes
- Blown fuse or faulty relay feeding the gas ECU
- Loose, corroded or water-damaged connector at the gas ECU
- Broken or chafed communication wiring (CAN or dedicated serial line) between gas ECU and engine ECU
- Failed gas ECU (after other causes eliminated)
Fault status
Status
Gas ECU fault — communication, power/ground, wiring issue or internal gas-ECU failure detected. Further diagnosis required.
Repair difficulty: Medium
Diagnostic time: 1.5-4 hours
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