What does it mean?
P1621 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Immobilizer Code Word/ID Number Not Registered .
Symptoms
- Engine cranks normally but never fires, or fires and stalls after a second or two
- Immobilizer or key symbol lit or flashing in the instrument cluster
- MIL (check engine lamp) on with a stored security related code
- Starts with one key but not with another
- Remote locking still works while starting does not
- Fault may be intermittent and worse in cold or damp conditions
Likely causes
- Engine module or immobilizer module replaced and never matched to the vehicle
- Key not learned to the system, or a copied key without a valid transponder
- Immobilizer data lost after low voltage or a failed programming session
- Faulty transponder antenna ring around the ignition lock, or a loose connector on it
- Communication fault between the immobilizer or cluster and the engine module
- Weak, damaged or water-soaked transponder inside the key
- Another transponder or fob on the same key ring interfering with the reading
How to fix it
- Try a second known-good key first; that alone separates a key problem from a system problem
- Read the immobilizer and engine modules and identify which side reports the missing registration
- Check battery voltage, because low voltage blocks most matching procedures
- Watch the key recognition status in live data while cranking to see whether the transponder is being read
- Inspect the antenna ring and its connector at the ignition lock, and remove spare keys and metal objects from the key ring
- Perform the correct component matching so the module and key are registered to the vehicle
- If matching fails with good voltage and a good key, test the communication between the modules before condemning either one