U0301 means a module on the network found the engine control module's software incompatible with its own. It belongs to the U03xx block that the SAE reserved for software incompatibility between modules, distinct from the U01xx and U02xx lost-communication codes. The engine module is still talking; what it says does not match what the other module was built to accept. The code is usually stored in the module that complained, most often the transmission control module.
Severity: moderate. Can you keep driving? Usually, with limits. Modules that cannot agree fall back to defaults: a transmission may hold a gear or shift harshly, a hybrid system may limit power, start-stop may be disabled. The engine itself usually runs. The fault will not clear on its own, because nothing about driving changes a software version.
Where it comes from
Honda documents the mechanism in two bulletins for the tenth-generation Civic:
- Service Bulletin 16-028 (revised March 2017), 2016 Civic 1.5 L turbo with rpm surging at highway speed: the fix is a transmission control module software update, and the bulletin warns that depending on the current software in the PCM, after updating the TCM you may get a U0301 software mismatch, so the PCM must be updated to match.
- Service Bulletin 17-012 (February 2017), 2016 to 2017 Civic with CVT that will not come out of Park or will not start with U0038 or U0101: the repair replaces or updates the TCM, and again on 2016 models you may set U0301 after the TCM is replaced, depending on the PCM's software.
Volkswagen's technical tips for 2013 to 2018 gasoline models after an ECM replacement describe U0301 the same way: "software incompatibility with ECM," resolved by bringing the replacement's software to the level the rest of the vehicle expects.
The pattern: one module was updated or replaced, its partner was not.
Causes, in the order they are found
A transmission or other module updated or replaced without matching the PCM, or a PCM replaced without matching the rest.
A used module from another vehicle with a different software level.
An interrupted programming session that left one module on a partial build.
Aftermarket tuning that changed one module's software identity.
A network fault that corrupts the version handshake, rare, and usually accompanied by lost-communication codes.
A failed module, last.
A diagnostic order
- Ask what changed. A recent TCM, PCM, hybrid or body module replacement or update, a dealer visit, or a tune, is the answer until proven otherwise.
- Read which module stored U0301. That module is the one complaining; the PCM is the one it is complaining about.
- Compare software levels of both modules against the current release with the factory tool or an equivalent.
- Update the module that is behind. In Honda's cases, the PCM after a TCM update; in VW's, the new ECM. Keep battery voltage steady with a charger during programming.
- Run any required learn procedures afterwards (transmission adaptations, throttle relearn).
- Clear the code and verify through a full drive. If it returns with both modules on current software, look for network faults, then at the module.
Frequently asked questions
Can I drive with a U0301 code?
Usually, with reduced function: the transmission or another system falls back to defaults. It will not fix itself. Have the software levels checked and matched.
Why did U0301 appear after my transmission module was replaced?
Because the new or updated transmission module runs a software build the engine computer was not on. Honda's Civic bulletins state that updating or replacing the TCM can set U0301 depending on the PCM's software, and the fix is to update the PCM to match.
Will replacing the engine computer fix U0301?
Almost never. The code is a version mismatch, and the fix is programming one module to match the other. A replacement PCM at the wrong software level sets the same code.
How much does it cost to fix U0301?
A software update at a dealer is typically an hour of labor. Independent shops with factory-level tools can do it on many makes. No parts are usually required.
Related codes
- U0300: internal control module software incompatibility.
- U0302 / U0303 / U0304: software incompatibility with the transmission, active suspension and gear shift modules; see U0304.
- U0101: lost communication with the TCM, stored alongside in Honda's 17-012 case.
- U0100: lost communication with the ECM/PCM; see U codes and module communication.
The code number itself tells you the family and the domain; how to read an OBD2 code number explains what each character means.