OBD-II Code P0605 — What It Means & How to Fix It

What Does This Error Mean?

P0605 means there is an error in the read-only memory (ROM) inside your engine control module (ECM/PCM). ROM is where the engine's programming — all its instructions for fuel delivery, ignition timing, and emission controls — is permanently stored. When the ECM detects that its own ROM has become corrupted or fails a self-check, it stores P0605. This is a serious code. Your car may run very poorly, go into limp mode, or refuse to start at all.

Affected Models

All vehicles 1996+
Common in GM vehicles 1996-2010
Common in Ford and Lincoln vehicles
Common in Chrysler, Dodge, Jeep, and Ram vehicles
Can affect any make or model if the ECM loses power during a firmware update

Common Causes

Corrupted ECM programming — often caused by an interrupted or failed software update
Internal ECM hardware failure — the ROM chip itself has degraded or failed
Extreme voltage spike (from a jump start or charging accident) that damaged ECM memory
ECM exposed to water or severe physical damage
Battery disconnection or power loss during a dealer ECM reprogramming procedure

How to Fix It

First, try clearing the code and restarting the vehicle. In some cases, a one-time power glitch can trigger P0605 and a successful restart clears it permanently. This is rare but worth trying first.

If the vehicle will not start or runs extremely poorly, skip this step and move directly to professional diagnosis.
Check battery voltage and condition. A weak battery that drops below 9 volts during cranking can cause ECM memory errors. Test the battery under load and replace it if it fails.

A battery test at any auto parts store is typically free. A voltage reading alone is not enough — you need a load test.
Check for any history of recent ECM programming. If the vehicle was recently at a dealer for a software update, an incomplete update may have corrupted the ECM. Return to the dealer and describe the issue.

Dealers are typically responsible for ECM damage caused by a failed programming procedure.
Have a professional attempt ECM reflashing (reprogramming). A shop with J2534 programming equipment or factory-level scan tools can often restore the ECM software without replacing the module.

Reflashing is much less expensive than full ECM replacement. Try this before replacing the module.
If reflashing fails, the ECM hardware has failed and must be replaced. Ensure the replacement ECM is properly programmed to your vehicle's VIN and options before leaving the shop.

Some vehicles require additional steps after ECM replacement, such as anti-theft system re-learning or idle relearn procedures.

When to Call a Professional

P0605 almost always requires professional intervention. A dealer or qualified shop with factory scan tools can attempt to reflash (rewrite) the ECM software. If the hardware has failed, the ECM must be replaced. ECM replacement costs range from $200 to $1,200+ depending on the vehicle. Replacement ECMs often require programming to match your vehicle's VIN — this adds $100 to $300 to the job. Do not attempt to replace the ECM yourself unless you have access to the programming equipment.

Frequently Asked Questions

What is the difference between P0605 and P0601?

P0601 is a RAM memory check sum error — it relates to the working memory the ECM uses while running. P0605 is a ROM error — it relates to the permanent programming storage inside the ECM. Both are serious, but P0605 (ROM error) is often harder to recover from without reflashing or replacement.

Can a jump start cause P0605?

Yes. Improperly jumping a car (hooking cables up backwards, or jumping a dead battery with a high-powered jump pack at full power) can send a voltage spike into the ECM that corrupts or damages ROM. Always connect jump cables in the correct order: red to dead positive, red to good positive, black to good negative, black to unpainted metal ground on the dead car.

Will my car pass emissions inspection with P0605?

No. P0605 is a serious diagnostic trouble code that will cause an immediate emissions test failure in any state with OBD-II testing. The code must be resolved before the vehicle can pass inspection.