1. Transmission code, not the model name
The model name tells you almost nothing. A single model year is routinely sold with two or three different automatics depending on engine and market: a ZF 8HP, an Aisin six-speed, a dual-clutch, a CVT. They are not interchangeable, and the casings sometimes look similar from underneath.
The code is on the data plate and usually repeated on a label on the casing. Ask for a photograph of that label, not a verbal confirmation.
2. Is the torque converter included?
Converters are frequently sold separately, and a gearbox quoted without one is not the same product as a gearbox with one. Beyond the price difference, converters hold fluid and wear material from the donor vehicle — a used converter from a failed gearbox is a contaminant you are about to install.
Confirm in writing: converter included, converter not included, or converter new.
3. Mechatronic unit and software
On modern automatics the mechatronic unit — the valve body with its control electronics — is the most failure-prone and most expensive sub-assembly. Two questions: is it included, and has it been coded to the donor vehicle?
If it is vehicle-coded, expect to need programming after fitting, which means a workshop with the right diagnostic platform. Budget for that before you buy, not after.
4. Donor mileage, and how it is documented
A gearbox from a 60,000 km car and one from a 260,000 km car can look identical on a pallet. Ask for the donor's mileage and how it was established — odometer photograph, auction record, or "believed to be". The answer tells you as much as the number.
5. Fluid and pan condition
Fluid colour is a crude but honest signal. Dark, burnt fluid means heat; a pan full of friction material means the clutches have been giving up for a while. A unit that has been steam-cleaned externally tells you nothing — ask for a photograph with the pan off, or of the drain plug magnet.
6. Adaptations and coding after fitting
A used automatic rarely works perfectly on first start. Shift adaptations learned by the previous unit need resetting, and on some platforms the transmission must be re-taught a new clutch fill volume. This is normal. What is not normal is a seller claiming a used gearbox needs no diagnostic work at all — that claim usually means they have not fitted one.
7. Known failure modes by family
- ZF 8HP — generally robust; the common failures are the mechatronic sealing sleeve and the bridge seal, both cheap parts requiring the valve body out.
- Aisin six-speed — durable; watch for torque converter shudder and worn valve body bores on high-mileage units.
- Dual-clutch (wet) — mechatronic solenoids and clutch pack wear; a used unit with unknown clutch life is a gamble with a known price.
- CVT — belt and pulley wear, and heat sensitivity. A used CVT without a documented service history is the riskiest purchase on this list.
Knowing which of these applies changes what to ask for. On a ZF, ask whether the sealing sleeve has been done. On a CVT, ask for service records or walk away.
8. What is actually in the crate
Gearboxes ship with or without: torque converter, mounts, cooler lines, wiring harness, selector lever, transfer case. Each omission is a part you will buy separately at retail, and the most expensive omission — the transfer case on an all-wheel-drive car — can double the bill.
Ask for a photograph of the complete assembly as it will ship, not of the gearbox alone.
9. Warranty and return freight
The part that matters is not the warranty length, it is who pays the freight. A 12-month warranty that requires you to return a 90 kg gearbox at your own cost across an ocean is not a warranty in any practical sense.
Confirm: warranty period, what it covers (function only, or function plus labour), and who pays return freight in each direction.
The request that gets a good answer first time
- VIN, photographed.
- Transmission code, photographed from the data plate or casing label.
- Donor-side photograph of the complete unit including converter.
- The fault you are chasing, with codes.
- Delivery country and postcode, so freight is quoted with the part rather than after it.
With those five, a quote is a quote. Without them, it is an estimate that will be revised — usually after you have paid.