What Is P0606?
P0606 indicates an internal processor fault within the ECU (Engine Control Module) or PCM (Powertrain Control Module). The ECU has self-diagnostic capability and when it detects that its own processor is not functioning correctly, it stores P0606 as a last-resort warning.
Like P0601, this is a high-severity fault. However, not every P0606 means the ECU needs replacing. Low voltage events, poor earths, and even static discharge can cause the processor to flag this code temporarily. Thorough checks of the power supply circuit should always precede ECU condemnation.
Common Symptoms
- Engine management light on
- Engine may not start
- Multiple stored fault codes across different systems
- Erratic or unpredictable engine behaviour
- Loss of communication with ECU via OBD port
- Engine cuts out without warning
Common Causes
How to Diagnose P0606
Record All Fault Codes
Document every code stored across all systems before doing anything. If the ECU has genuinely failed, you may lose the ability to read codes entirely. Note all codes for reference.
Test ECU Power Supply
Measure voltage at the ECU main power connector with engine running. Should be stable at battery voltage (13.8–14.4V). Fluctuating voltage can cause processor faults.
Test All ECU Earth Connections
Using a multimeter, measure the voltage drop between each ECU earth pin and the battery negative. More than 0.5V on any earth indicates a high-resistance earth causing the fault.
Try a Battery Disconnect Reset
Disconnect the battery for 15–30 minutes. Reconnect and attempt to start. On some vehicles, a processor that has locked up can be reset this way. Clear codes and observe if P0606 returns immediately.
Attempt ECU Reflash
A dealer or ECU specialist can attempt to reflash the ECU firmware. This resolves P0606 when the cause is firmware corruption rather than hardware failure.
Replace ECU
If all else fails, the ECU processor has physically failed and the unit needs replacement. This requires coding to the vehicle — do not attempt with a used ECU without specialist support.
Mechanic's Corner — PCM Processor Faults
P0606 is a serious code but it's worth exhausting every other possibility before condemning the ECU. In workshop experience, wiring issues account for a surprising number of P0606 cases — particularly corroded multi-plug connectors on the ECU itself, which are exposed to water ingress on many Ford, Vauxhall, and VW models. Remove the ECU connector, inspect carefully for green corrosion or bent pins, and apply a thin coat of dielectric grease on reassembly.
If the fault persists after checking power supply and connectors, send the ECU for specialist testing before buying a replacement. Faulty ECUs can often be repaired for £80–£150, versus £300–£800 for a new unit.
Verdict
Test the power supply and earth connections to the ECU thoroughly before replacing it. A reflash resolves firmware-related P0606. Physical ECU failure requires replacement and coding.
