Over-the-air firmware updates: what changes in your car while you sleep

Until recently, changing control software meant a workshop visit and a connection to factory equipment. Now some cars update themselves — overnight, in a car park, over mobile data or home Wi-Fi.

For an owner that is convenient and also creates a new situation: the car may work differently today than yesterday, and the reason is not a fault.

What actually gets updated

Two distinct levels matter here, and people confuse them constantly.

Infotainment and navigation. The most common case. The interface changes, maps change, app support changes. It does not affect how the car drives, and the risks are minimal.

Powertrain and chassis modules — engine, transmission, brakes, driver assistance. This is where it gets interesting: calibrations change, shift logic changes, system thresholds change. The car is physically identical and drives differently.

The second level is not available from every manufacturer or on every model. But where it exists, it gets used.

What changes in the data

The most direct trace of an update is in Mode 09, the “Vehicle Information” service:

• CALID — the calibration identifier, effectively the software version.
• CVN — the checksum of that software.

Both change after an update. The fields are covered in their own article.

The practical takeaway: record CALID and CVN before an update. It takes a minute and afterwards lets you say precisely when the car’s behaviour changed.

The same mechanism is used to detect non-original calibrations — see detecting an ECU remap. The only difference is who made the change.

Side effects nobody warns you about

Updating powertrain modules almost always has consequences that look like faults.

Adaptations reset

The module spent years learning your specific car: throttle position, fuel trims, transmission shift points. After an update some of those values are zeroed.

What you will feel: for the first days the car behaves slightly differently — hunting idle, harsher shifts, higher consumption. That is not a failure, it is relearning.

How long it lasts: typically 60–200 miles of ordinary driving.

Readiness monitors reset

Updates are frequently accompanied by clearing fault memory — and the monitors go with it. All not ready.

Consequence: if a test is coming up, you will fail it until a full drive cycle completes. The procedure is in the drive cycle guide, and test requirements in the inspection guide.

Practical takeaway: do not schedule an update a week before a test.

Codes appear that were not there before

A new software version may use different test thresholds. A fault that previously sat inside tolerance is now outside it — and a code appears.

That does not mean the update broke something. It means the update pays closer attention.

What to do before and after

Before:

1. Record CALID and CVN.
2. Read and save current codes, pending included.
3. Confirm the battery is healthy: 12.4–12.7 V overnight. A voltage sag mid-flash is the worst possible scenario.
4. Do not schedule an update before a long trip or an inspection.

After:

1. Compare CALID and CVN — establish that a change happened.
2. Read codes: whatever appeared immediately is a consequence of the reset, not a failure.
3. Give the car 150–200 miles to relearn before drawing conclusions.
4. Check the readiness monitors.

When to be concerned

Not everything that appears after an update passes on its own:

• A code returns after clearing and a full cycle. That is a real fault, not a reset artefact.
• Consumption changed noticeably and has not recovered after 200 miles.
• Limp mode — covered separately.
• A module vanished from the roll call. The most alarming sign: the flash may have been interrupted.

That last one is a reason to contact the manufacturer rather than hunt a fault yourself.

What this means when buying a car

A new layer of questions for the seller:

• Whether all updates are installed — visible in the manufacturer’s app on some models.
• Whether the modules carry factory CALIDs or something else.
• Whether the previous owner is still on the paired accounts list — covered in the cybersecurity article.

The general inspection protocol is in the used car guide.

In short

• Two different levels update: infotainment and powertrain modules. Do not conflate them.
• Record CALID and CVN before an update — one minute, invaluable later.
• Reset adaptations cause “odd” behaviour for 60–200 miles. That is normal.
• Monitors drop to not ready — do not update before an inspection.
• A code that returns after a full cycle is a fault, not an artefact.

Download the app — Mode 09, codes, and monitors are read with an ELM327 adapter over Bluetooth SPP on Android.