P1612 Code: Engine Computer to Immobilizer Communication Failure (Manufacturer Code)

What the P1612 code means

The anti-theft system on a modern vehicle works as a chain with three links. The key carries a transponder that answers the immobilizer antenna; the immobilizer module verifies that code; and then that module and the engine computer swap their own authentication before fuel injection is allowed.

That last step is more elaborate than it looks. In many systems the computer sends the immobilizer a random number, both modules run the same calculation on it, and only if the results match is fuel enabled. That mechanism stops anyone from plugging in a foreign module and starting the vehicle.

The P1612 code is stored when that dialogue between modules fails. On Nissan and Infiniti vehicles it is described as a fault in the chain between the computer and the immobilizer; on General Motors group vehicles, as an authentication error of the immobilizer module.

The distinction from P1614 is what organizes the diagnosis. That code says the immobilizer could not read the key; this one says the key could be read but the two modules did not agree. In the first case you suspect the key and the antenna; in this one, the wiring between modules, the programming or one of the two modules.

One detail is worth knowing because it can save you from losing the vehicle. Many anti-theft systems limit the number of failed authentication attempts: after several, the module goes into a temporary lockout and accepts no more tries for a few minutes. Insisting in that state gets you nothing and can extend the lockout.

The right move is to turn the ignition on, wait the time the manufacturer specifies with the immobilizer light on, and only then try again. That detail explains quite a few cases where the vehicle starts on the third visit to the shop with nobody having done anything new.

P1612 code symptoms

• The engine cranks with the starter but does not fire.
• Immobilizer light on or blinking.
• Check engine light on.
• No injector pulse while cranking.
• The fault appears after a module was replaced or reprogrammed.
• The engine starts and shuts off after a few seconds.
• The behavior is identical with any key.
• After several failed attempts the system stops responding for a few minutes.

Likely causes of the P1612 code

1. Module replaced without running the programming procedure. A very frequent cause.
2. Used engine computer not synchronized with the vehicle immobilizer.
3. Damaged wiring or connector between the two modules.
4. Weak battery, whose low voltage keeps the authentication from completing.
5. Failed immobilizer module.
6. Engine computer with an internal fault.
7. Rusty body grounds.
8. Interference from installed accessories on the communication line.

How to diagnose the P1612 code step by step

1. Check whether the behavior is the same with another key. If it is, the problem is not the key and this code fits; if it changes, look at P1614.
2. Ask whether any module was replaced recently. An unprogrammed computer or immobilizer explains the code with no further investigation.
3. Test the battery by measuring voltage during cranking: below nine volts the authentication does not complete.
4. Inspect the wiring and connectors between the immobilizer and the computer.
5. Check for communication codes in other modules.
6. Clean and tighten the body grounds.
7. Run the synchronization procedure with a tool that has brand coverage.
8. Confirm the exact definition for your brand, since it is a manufacturer code.
9. Do not keep cranking repeatedly. Many systems lock out after several failed attempts: turn the ignition on, wait and try again.

How to fix the P1612 code

• Run the programming and synchronization procedure if the computer or the immobilizer was replaced.
• Replace or recharge the battery if its voltage is insufficient.
• Repair the wiring and connectors between the two modules.
• Clean and tighten the body grounds.
• Replace the immobilizer module if it does not respond to authentication with the wiring good.
• Replace and program the engine computer only after ruling out everything above.
• Remove accessories that interfere with the communication line.
• Respect the waiting time with the ignition on before you repeat the start attempt.

Programming immobilizer modules requires a tool with brand coverage and, in many cases, proof of vehicle ownership.

Technical reference data

• Authentication is usually based on a random number that both modules calculate and compare.
• Without a match, the computer does not enable fuel injection or the fuel pump.
• Minimum battery voltage to complete authentication: above 9 volts.
• Replacing the computer or the immobilizer requires a programming procedure afterward.
• P1614 corresponds to the link between the immobilizer and the key; this one, to the link between modules.
• Anti-theft systems often lock out temporarily after several failed authentication attempts.

Brand-specific notes

The test that guides everything is to try another key. If the behavior is identical with both, the problem is not the key and this code fits; if one key starts the engine and the other does not, it is a key issue and belongs more to P1614.

The most repeated cause is a used computer installed without synchronizing it. That operation needs a tool with brand coverage, and in many cases cloning is not enough: the official programming procedure has to be run.

P1612 on Nissan and Infiniti

The code is described as a fault in the chain between the computer and the immobilizer.

P1612 on General Motors Vehicles

The code is described as an authentication error of the immobilizer module.

Frequently asked questions about P1612