Returning the ECU to factory software

Reset to Stock puts the factory software back into the engine control unit. Tunes, system and fault deletes and other tuners' edits are gone, and the ECU runs on its factory calibration again. For tuners: send us the current file and get the factory file of the same version, ready to flash, on average within 30–60 minutes.

When cars go back to factory software

• Selling the car. The buyer asks for factory software, and a modified file shows up in a stock check.
• Dealer visits and warranty. Owners return to stock before a dealer visit or a warranty claim. The manufacturer may also update the software during a service, in which case the ECU ends up with a newer factory version.
• Software checks at inspection. In California, a car that fails Smog Check with "Modified Software" can only be retested after its software is restored to the manufacturer's version or a CARB-approved one.
• A bad or unknown tune. Flat spots, fault codes, nobody knows what was written. It is easier to go back to stock and build the solution again from the original.
• Before a new tune. Even if the previous software worked, the new one is better built from the original. Viezu gives its dealers the same advice: on a previously tuned car, return to stock first, then write the new map.
• Hardware back in place. A new particulate filter, catalyst or EGR valve has been fitted, and the ECU has to control and monitor it again.

Where the factory file comes from

A random "stock file for this engine" will not do. You need the original for that exact hardware and software version, and the three possible sources are not equally good.

• A backup of this ECU taken before the first write. This is the best option, because it is exactly what was on the car. A full bench backup covers all memory areas, and Autotuner names returning a vehicle to its original state as one of its two purposes. If another tuner took the backup, make sure first that it really is the original: a file named ori is often already modified.
• An original from a database, matched by hardware and software number. Without a backup, the factory file of the same version is looked up in a database, for example in the stock file section.
• A factory update. A newer version from the manufacturer is stock as well. Autotuner's FRF selection window for VAG works this way: flashing the original returns the vehicle to its original state, and earlier edits (tuning, patches, unlock) are erased.

A file from a different ECU, or a generic "stock" map from a tuning app, will not do. Forum threads describe cases where such a "stock" file failed California's inspection because the CAL ID and CVN did not match what was expected for that car.

Which software version to restore

Usually the one that was installed before the tune. There are a few forks in the road, though:

• The manufacturer has released an update. A newer factory version for the same hardware is stock too. If the customer is heading to the dealer anyway, the ECU may get updated during the service.
• The car has had a recall update. The file read before the tune may be older than the recall software. Autotuner warns about exactly this: after a Dieselgate update, the list may show a previously installed file. Agree with the owner which version goes back in.
• No original exists anywhere. No backup and no reference of the same version: the remaining route is a factory update, for example at a dealer.

ECUs that were unlocked for tuning need a separate thought. Writing the factory file can lock them again, and the next tune will then need another unlock, sometimes on the bench. EcuTek even splits this into two functions: Return to Stock leaves the ECU unlocked, while Reset to Factory State relocks it and removes the EcuTek data.

What comes back and what stays

A return to stock means writing the complete factory file rather than undoing individual edits. Another tuner's changes can sit in the maps, in the fault table and in the code, and it is easy to miss one by hand. The complete original also brings back the factory checksums and CVN.

The last row of the table matters to owners. After the return the ECU content is factory, but on many ECUs the fact that it was reflashed stays in the programming history. On VAG, for example, a scan tool shows the number of programming attempts, and GM controllers kept a reprogramming history back in the 2000s. Nobody can honestly promise that "the dealer won't see a thing".

One more thing: the return covers the ECU whose file you sent. If the gearbox (TCU) or a separate SCR unit (DCU) was modified too, each of them goes back to stock separately.

The hardware has to be in place

Factory software expects the factory set of components. The tune had switched off the checks on those parts, and after the return they run again: if something has been removed, a fault appears as soon as the ECU gets to checking that part.

If components are already gone and the car needs stock software, put the hardware back first. Otherwise the customer gets fault codes instead of factory software, and on some cars reduced power as well.

What to do before and after flashing

• Read the ECU again before writing and confirm the software version. The manufacturer may have updated it without the owner knowing, and writing a file of the wrong version can leave the ECU unable to talk over OBD, the car unable to start and the recovery in the manufacturer's hands.
• Connect a power supply or battery charger. Writing stock software is a reflash like any other, and a sagging battery can interrupt it.
• Clear faults in all modules, including the ones outside the engine.
• Check the restored systems in live data: DPF regeneration, EGR operation, AdBlue.
• Let the car drive before an inspection. OBD readiness status counts from the moment faults were cleared, and incomplete monitors are a reason to fail, for example at California's Smog Check.

How to order a return to stock

1. Read the ECU with the tool you use. If a pre-tune backup exists, attach it too.
2. Upload the file to the portal and select RESET TO STOCK.
3. Get the factory file, ready to flash, on average within 30–60 minutes.
4. Flash the file, clear faults in all modules and check that the hardware works.