Internal Control Module Read Only Memory (ROM) Error
The Powertrain Control Module (PCM) continuously runs self-diagnostic tests on its internal memory, specifically the Read Only Memory (ROM). This code is set when the PCM detects a checksum error or failure within its ROM, indicating an internal fault in the module itself.
A ROM error means the engine computer is failing internally. This can lead to unpredictable engine behavior, stalling, or a complete no-start condition, making it unsafe to drive.
Common symptoms
- Illuminated Check Engine Light
- Engine cranks but will not start
- Engine stalling or dying unexpectedly while driving
- Erratic or harsh transmission shifting
- Multiple unrelated warning lights illuminating on the dashboard
Likely causes
- Internal failure of the Powertrain Control Module (PCM)
- Corrupted PCM software or failed aftermarket tune/flash
- Poor power or ground connections to the PCM
- Voltage spikes from a failing alternator or improper jump-start
- Water intrusion or physical damage to the PCM
What to check before replacing parts
Check battery voltage and charging system health.
Look for: Ensure the battery rests at 12.4-12.6V and the alternator outputs 13.5-14.5V without excessive AC ripple, as severe voltage issues can cause false PCM codes.
Inspect the PCM wiring harness and connectors.
Look for: Look for loose pins, corrosion, or signs of water intrusion at the PCM connectors that could disrupt power or ground.
Perform a voltage drop test on PCM power and ground circuits.
Look for: A voltage drop of more than 0.1V on grounds or 0.2V on power feeds indicates high resistance that must be repaired before condemning the PCM.
Check for Technical Service Bulletins (TSBs) and available software updates.
Look for: Determine if the manufacturer has released a software update or reflash procedure that addresses a known false P0605 code.
Clear the code and perform a test drive to see if it returns.
Look for: If the code returns immediately or after a short drive, and all power/ground circuits are perfect, the PCM itself has failed and requires replacement or reprogramming.
This page explains the generic P0605 definition and is general reference information, not a diagnosis for your specific vehicle. Actual causes, severity, and repairs vary by make, model, engine, and mileage — always confirm with a scan tool and a qualified technician before replacing parts.
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