No Start | Immobilizer Codes B2796 & B2799

🚗 2001 Lexus LS430

• The customer lost a key, few locksmiths have tried to program another key, but couldn’t.

🩺 Diagnosis

➱ 📚 2001 Lexus LS430 Immobilizer Manual

Surprisingly, this car had two issues that needed to be solved before it started.

1. No Communication Issue (Bad Over Head Junction Box)

First, there was no communication with any of the computers located on the communication line (MPX1 on the diagram).

• Disconnecting all ECU’s that are connected to that communication network helped to find a bad Over Head Junction Box (a common problem). Disconnecting it restored communication with all other ECU’s.
• 

2. Key Programming Issue (Bad Immo Box)

Second, we needed to program a key, but it wouldn’t want to program it.

• Performed Immobilizer Reset (5 seconds with AVDI) which cleared all the keys. At this point it should have entered in automatic registration mode, automatically registering any inserted key, but it wouldn’t enter it.
  • Note, to forcibly close automatic registration mode
    • turn the ignition switch ON/OFF alternately more than 5 times in 10 sec.
• Had a code B2796 No Communication in Immobilizer System.
  • 
  • If there was good communication, but the key was not programmed, it would show a code B2795 Unmatched key code.
  • This means that we need to focus on the communication between Immobilizer Box and Transponder Key Amplifier.
• Pin 1 of the Transponder Key Amplifier would have anywhere between 0V and 7V when the key was inserted in the ignition.
  • With a good Immo Box it would have 12V for a split second (see waveforms below).
  • Tested power and ground to the Immobilizer Box, all ok.
  • Tested wires between Immobilizer Box and Transponder Key Amplifier.
  • Turned out, Immobilizer Box was bad (bad transistor inside).

Testing Transponder Key Amplifier with an Oscilloscope

• Bad Immo Box	Good Immo Box

Immobilizer’s Live Data Comparison

• Bad Immo Box (original)	Good Immo Box (used, key is not programmed)	Good Immo Box (used, key is programmed)
  Master key – “Unregistered” Sub key – “Unregistered” Response – “Not Good” Transponder M-Code – “0” Transponder S-Code – “0” Immobilizer Box has been reset, so it should have 0 keys stored.	Master key – “Unregistered” Sub key – “Unregistered” Response – “OK” Transponder M-Code – “3” Transponder S-Code – “1” This is good Immobilizer Box from another car, with few keys programmed into it already.	Master key – “Registered” Sub key – “Unregistered” Response – “OK” Transponder M-Code – “1” Transponder S-Code – “0” This is good Immobilizer Box from another car with all keys cleared (Immobilizer Reset) and 1 master key programmed.

B2799 Immobilizer System Malfunction

If ECM or Immobilizer Box is replaced, or reset, ECM will store code B2799 Immobilizer System Malfunction. To synchronize ECM and Immobilizer Box, jump OBDII pins 4 and 13 for 30 mins.

Immobox Repair

If you have another Immobilizer Box, you can try to replace the transistor that supplies power to the Transponder Key Amplifier (pin 1). This way you wouldn’t need to bother with key programming.