// Official meaning
Mercedes code: "Drive authorisation not carried out / immobiliser signal not received" — the transmission control unit cannot authenticate with the vehicle’s FBS4 security system.
// On the bench
This is the code that appears after someone fits a used TCU from a breaker. The donor unit still holds the donor car’s cryptographic keys and adaptation data, so your EIS and engine ECU refuse to recognise it — the car cranks but will not drive, or will not crank at all. Another used unit will not fix it; the TCU has to be reset to virgin state and married to your car, which requires an original FBS4 licence. We hold one and do the whole job in-house.
How it shows up
- No drive after a TCU replacement — cranks but will not move
- Transmission stuck in limp mode or in Park/Neutral
- Transmission warning or "Visit Workshop" message
- P051300 stored and returning after every clear
What fixing it involves
Virgin-state reset and FBS4 adaptation of the TCU — used units wiped of donor data, programmed to your vehicle and bench-tested before dispatch. A fraction of the £2,000–£4,000+ dealer route of a brand-new unit plus programming.
Frequently asked questions
I fitted a used TCU and got P051300 — is the unit faulty?
Probably not. It is almost certainly fine — it is just still married to its donor car. FBS4 ties the TCU to the EIS and engine ECU, so it needs a virgin reset and proper programming before your car will accept it.
Do I send the whole car or just the TCU?
For most jobs, just the TCU — we bench-programme and test it, then send it back ready to fit. A mobile on-vehicle service is available too if you would rather we came to you.
Is it the TCU or the gearbox itself?
A proper scan tells you: electrical faults, communication errors and P051300 point at the TCU; mechanical failures present as noise, vibration or leaks. Share your codes over the phone and we will advise honestly.