Enter a 5-character code such as P0420, P0301, or P0457.

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Codes starting with P are engine and transmission, B are body, C are chassis, and U are network communication. A 0 as the second character means a generic code shared by all makes; a 1 means a manufacturer-specific code, so its meaning depends on your brand.

A steady check engine light usually means get it checked soon. A flashing check engine light means an active misfire: reduce speed and stop driving as soon as it is safe.

That doesn't look like an OBD-II code

A valid code is one letter (P, B, C, or U) followed by four characters, for example P0420.

U0100 Lost communication with ECM/PCM

Can I drive with U0100?

Drive with caution

Stop if the engine stalls, hesitates or won't restart.

The car's modules can't talk to the engine computer. The car may stall or refuse to start.

What does U0100 mean?

Other control modules on the vehicle network stopped receiving messages from the engine computer.

What causes U0100?

Weak battery or low voltage
Loose or corroded connector at the engine computer
Damaged network wiring
Blown fuse
Failed engine computer

How much does it cost to fix U0100?

$100 – $1,500

Battery or connector fix: $100–$300. Engine computer replacement: $500–$1,500.

Confirm the code yourself

A low-cost scanner lets you read the code, clear it, and see if it comes back before paying for a diagnosis.

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