What Does P1621 Mean?
The immobilizer system has detected a mismatch between stored security codes in the PCM and the key transponder. This indicates a security system communication failure preventing engine start authorization.
Safety Warning
Vehicle will not start or will start briefly then shut down. No immediate safety risk but creates potential stranding situation.
Common Causes
35%
PCM replacement or reprogramming without proper immobilizer synchronization
25%
Damaged or incorrect key with failed or missing transponder chip
20%
Immobilizer antenna ring failure or damaged wiring around ignition cylinder
12%
Low vehicle battery voltage causing memory corruption during key learning
8%
Security module or body control module communication failure
Diagnostic Steps
1
Step 1: Attempt to start vehicle with all available keys to determine if issue is key-specific or system-wide
2
Step 2: Check battery voltage - must be above 12.4V during immobilizer learning procedures
3
Step 3: Use factory scan tool to read immobilizer system status and verify communication between PCM, security module, and BCM
4
Step 4: Perform key learning/programming procedure per manufacturer specifications using dedicated security access tools
5
Step 5: If reprogramming fails, inspect antenna ring at ignition cylinder and test resistance (typically 100-200 ohms)
Estimated Repair Cost
$200 - $1,500
Parts + labor, varies by vehicle and location
The immobilizer system has detected a mismatch between stored security codes in the PCM and the key transponder. This indicates a security system communication failure preventing engine start authorization.
The most common cause of P1621 (Control Module Long Term Memory Performance/ Immobilizer Code Words Do Not Match) is: PCM replacement or reprogramming without proper immobilizer synchronization
Typical repair costs for P1621 range from $200 to $1,500, depending on the vehicle, location, and whether you do it yourself or go to a shop.
Vehicle will not start or will start briefly then shut down. No immediate safety risk but creates potential stranding situation.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to P1621 to identify the root cause.
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Quick Info
Category
Powertrain
System
Anti-Theft/Security System
Difficulty
Type
Manufacturer
Recommended Tools
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