OBD-II Code P1600 — What It Means & How to Fix It

What Does This Error Mean?

P1600 indicates a loss of communication between the PCM and an internal keep-alive memory (KAM) circuit. The PCM stores learned fuel, idle, and transmission data in KAM. When power to that memory is interrupted or the circuit fails, the PCM sets P1600. You may notice rough idle, poor fuel economy, or transmission shift issues after the code appears. This code is seen on many Toyota, Lexus, and some Nissan models.

Affected Models

Common in Toyota Camry and Corolla
Common in Toyota Tacoma and 4Runner
Common in Lexus ES and IS series
Common in Nissan Altima and Maxima on some platforms

Common Causes

Discharged or dead battery that wiped the PCM keep-alive memory
Faulty PCM power supply relay or blown fuse feeding the PCM memory circuit
Corroded or loose battery terminals interrupting PCM backup power
Internal PCM failure — memory chip or circuit board fault
Recent battery disconnection or jump-start that reset PCM memory

How to Fix It

Check all fuses related to the PCM, ECM, and EFI in both the under-hood and interior fuse boxes.

A blown EFI or ECU-B fuse is a common cause of P1600 and is the cheapest fix.
Inspect the battery terminals for corrosion. Clean and tighten them securely.

A loose negative terminal can intermittently cut power to the PCM memory circuit.
Test battery voltage with the engine off. It should read 12.4 volts or higher.

A weak battery that drops below 10 volts during cranking can corrupt PCM memory.
Clear the code and drive the vehicle through several normal drive cycles to allow the PCM to relearn.

If the battery was recently disconnected, the PCM simply needs time to relearn — the code may not return.
If the code returns after relearning, have the PCM tested or replaced by a qualified technician.

Internal PCM memory faults cannot be repaired at home — professional diagnosis is needed.

When to Call a Professional

PCM replacement ranges from $150 to $600 depending on the vehicle. Programming a new PCM adds $50 to $150 at a dealer or shop. If only a fuse or relay is involved, repair cost is under $20. Get a diagnosis before replacing the PCM — misdiagnosis is expensive.

Frequently Asked Questions

Will P1600 cause my car to fail emissions testing?

Yes — an active P1600 will illuminate the check engine light. Any active check engine light is an automatic emissions test failure in most states. Fix the root cause and clear the code before testing.

Can I drive with P1600?

The car will usually still start and drive. However, the PCM may not be managing fuel and transmission optimally. You may notice rough idle, hesitation, or poor MPG until the PCM relearns or is repaired.

Does disconnecting the battery cause P1600?

Yes, on some Toyota and Lexus models, disconnecting the battery can trigger P1600. In many cases the PCM simply needs to relearn its settings over several drive cycles. If the code keeps coming back after relearning, suspect a PCM power supply fault.