P0606: OBD-II code P0606 flags an internal PCM/ECM processor fault — the module has detected a problem in its own hardware or software self-test.

Symptoms

• Check engine light is on; the vehicle may run poorly, stall randomly, or refuse to start.
• Multiple unrelated fault codes may set together because the PCM can't reliably process its inputs.
• The code often will not clear — it returns immediately after a scan-tool clear while the internal fault persists.

Common causes

• A genuine internal PCM failure that has failed the module's built-in self-test on start-up.
• Corrupted PCM firmware from a botched reflash or a power loss during a programming session.
• Low or unstable battery voltage repeatedly tripping the module's internal watchdog into a fault state.

What to do?

Confirm battery voltage is 12.4 V+ at rest and 13.8-14.5 V running. Check PCM main power and ground with a DMM. If voltage is good and P0606 will not clear, the PCM has failed and needs replacement or professional rebuild, followed by programming with the vehicle's VIN and options.