Buying a Used ECU: Checklist for Numbers, Software Level, Immobiliser Type and Gearbox Variant

Transit Custom 2.2, SID208, ECU dead after a jump start. Two used ones on offer: BK21-12A650-AC with BK21-14C204-BPK, and BK21-12A650-AC with BK21-14C204-BLV. Same hardware, same price, and only one of them will drop in without a file. Buying a used ECU is a numbers job before it’s a wiring job, and the numbers you need aren’t always the ones the seller lists.

Match these, in this order

1. ECU type and hardware number. SID208, EDC17C46, CAS3, whatever it is, and the Bosch/Siemens/Delphi hardware reference. Wrong hardware means the plug fits and nothing else does.
2. Car maker part number, including the suffix. 03L997018B is not 03L997018. BK21-12A650-AC, not just 12A650. On BMW CAS3, 6135 9389116-01, including the -01 index.
3. Software level. The four-digit VAG version, the Ford 14C204 strategy (BPK against BLV above), the Bosch 1037/10SW, the PSA 96/98xxxxxx80 software number, the Nissan T-code. A used ECU one software level behind will run; several levels behind can mean limp mode, DPF regeneration complaints or a gearbox that shifts oddly.
4. Engine code and emissions variant. Euro 4 against Euro 5 on the same engine family is a different file and sometimes different hardware.
5. Gearbox variant. Manual, automatic, DSG. An EDC17 from a manual car in an automatic will run the engine and put the gearbox in emergency mode.
6. Injector class and adaptations. On EDC16/EDC17 the injector codes (IMA) live in the ECU; you’ll re-enter them from the injectors’ own labels after fitting. If the injectors have been changed before and nobody kept the codes, plan for that.
7. Immobiliser type. The one most sellers skip. See below.

Immo: what still needs a file after the swap

A used engine ECU almost always arrives married to another car. Options, by manufacturer:

• VAG immo 3/4: key adaptation with VCDS/VCP works on most, or a virgin file so the ECU learns on first start. Immo 5 and component protection need online ODIS or a dealer.
• BMW: DME and CAS share an ISN. Either align the ISN with a tool that can (Autohex, AVDI, ACDP on many) or write a virgin file into the EDC16CP35/EDC17 and let the car learn it. The catalogue carries VIRGIN DUMP files for exactly this: X3 E83 EDC16CP35 0281015241 1037390902, 730Ld E65 0281013253 1037387658.
• Mercedes: FBS3 cars (CRD2, CRD3, EDC16C2) take a virgin file, for example the Sprinter W906 CRD2 6519020100 6519032017 VIRGIN DUMP or the Vito W639 EDC16C2 A6111534979 0281011176. FBS4 cars cannot be virginised; the used ECU is useless unless it was never coded, so ask.
• PSA: ECU pairs with the BSI. Virgin file then Diagbox pairing, or an immo off file if the customer accepts that.
• Renault: UCH pairing with CLIP, or virgin.
• Ford: PATS. Parameter reset with IDS or FORScan, or immo off on the older SID and Delphi units.
• Hyundai/Kia: SMARTRA pairing; immo off files exist for EDC17C08, M7.9.8 and SIM2K-341.

What the seller must photograph

• The full label, straight on, in focus, every number legible. Not a listing photo of a similar unit.
• The connector pins (bent or green pins are a return waiting to happen).
• The case seams and the label edge. A resealed case or a lifted label means the box has been opened; that’s not always bad, but you want to know.
• For BMW CAS, Mercedes EIS and any body module: the VIN it came from, so you know it wasn’t already virginised or half-programmed by someone else.

Questions to ask before paying

Was it removed from a running car?

A box pulled from a running car has a known-good file inside. A box from a ‘non-runner, unknown fault’ might be the reason the car didn’t run.

Has it been flashed or tuned?

A tuned file, a DPF off, an immo off already inside: fine if you know, a diagnosis nightmare if you don’t. Ask for the ident from the seller’s tool if they have one.

What is the return policy if the numbers match but it doesn’t work?

Get it in writing. Fourteen days with the buyer paying return postage is common; ‘no returns on electrical’ is a reason to buy elsewhere.

What went wrong on the bench

The Custom above: the workshop bought the BLV unit because it was cheaper. It started, ran, and threw a gearbox comms fault, because the BLV strategy belonged to a different spec. A stock BPK file written into the BLV box (same hardware, same 12A650 number) fixed it, plus a PATS reset with IDS. Half a day lost that a label comparison would have saved.

When the used box needs a file, look under virgin files, immo off or the original files for the correct software level, and read our original vs virgin vs FRF guide if you’re unsure which one applies.