Weak or dead transponder key battery or a damaged key chip; Faulty immobilizer coil antenna around the ignition switch or push-button receiver; Poor electrical connection or corrosion at the immobilizer module connector; Failed immobilizer control module or its internal memory; PCM losing power, ground, or CAN (Controller Area Network) communication with the immobilizer; Aftermarket remote starter or alarm system interfering with the immobilizer circuit; Low vehicle battery voltage causing the immobilizer to drop out during cranking

DTC codes / Mazda codes / P1604

P1604 · MazdaImmobilizer System Fault

Q: What does code P1604 mean?

Immobilizer System Fault. P1604 on a Mazda means the engine computer (PCM) has lost communication with the immobilizer system, which is the anti-theft circuit that reads the key transponder and authorizes the engine to start. The PCM sets this code when it does not receive the expected validated key signal or when the immobilizer control module reports an internal fault, so the engine may crank but not run or may not crank at all.

P1604 on a Mazda means the engine computer (PCM) has lost communication with the immobilizer system, which is the anti-theft circuit that reads the key transponder and authorizes the engine to start. The PCM sets this code when it does not receive the expected validated key signal or when the immobilizer control module reports an internal fault, so the engine may crank but not run or may not crank at all.

SystemPowertrain

Code typeMazda specific

SeverityHigh

Fix difficultyProfessional

AreaComputer and output circuits

Symptoms you may notice

Engine cranks but will not start, or starts and stalls immediately
Security or key warning light flashing or staying on steadily
No communication with the immobilizer module when scanned
Intermittent no-start that clears after several attempts or a battery disconnect
Remote keyless entry or door lock functions behaving erratically
Often no symptoms besides the warning light if the fault is stored but not active

Common causes, most likely first

Weak or dead transponder key battery or a damaged key chip
Faulty immobilizer coil antenna around the ignition switch or push-button receiver
Poor electrical connection or corrosion at the immobilizer module connector
Failed immobilizer control module or its internal memory
PCM losing power, ground, or CAN (Controller Area Network) communication with the immobilizer
Aftermarket remote starter or alarm system interfering with the immobilizer circuit
Low vehicle battery voltage causing the immobilizer to drop out during cranking

How to diagnose and fix it

Scan all modules for stored codes and confirm whether the immobilizer module is communicating
Check battery voltage and load-test the battery, then clean and tighten the battery terminals
Try a second known-good transponder key to rule out a failed key chip
Inspect the immobilizer antenna coil and module connectors for corrosion or loose pins
Verify power and ground at the immobilizer module with a wiring diagram and multimeter
Perform the Mazda key relearn or immobilizer reset procedure with a capable scan tool
Replace the immobilizer module or antenna coil only after electrical checks pass, then reprogram keys

Why a Mazda cranks but will not start

A weak car battery is the most common trigger because the immobilizer drops out during cranking, so always load-test the battery before condemning the module.
Aftermarket remote starters and alarms are frequently spliced into the immobilizer wiring and cause P1604 even when the factory system is healthy.
P1604 does not always mean the immobilizer module is dead; a broken antenna coil ring or a bad key chip produces the same code.
Clearing the code without fixing the root cause usually results in a no-start again within days, so diagnose the communication fault first.
Some Mazda models store P1604 when the PCM itself loses ground, so check engine ground straps before replacing any module.

Can I keep driving with P1604?

Do not drive the vehicle if it will not start or stalls immediately, because the immobilizer has disabled the engine. If the car starts and runs normally with only a stored code, drive directly to a repair shop and avoid shutting off the engine in an unsafe location until the fault is repaired.

Tools that help

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Guidance is general and written for Mazda vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.