2019 Nissan Altima Auto Start

Alright, let's talk about the 2019 Nissan Altima's auto start system. It's a convenient feature, but understanding how it works under the hood can be crucial for troubleshooting, potential modifications, or even just appreciating the technology.

Understanding the Basics

The auto start system, officially known as Remote Engine Start (RES) in Nissan parlance, allows you to start your Altima's engine remotely using the key fob. This is especially useful for pre-conditioning the cabin in extreme weather – heating it up in winter or cooling it down in summer. However, it's more than just pressing a button. Several layers of safety and security protocols are built in.

Key Fob and RF Communication

The key fob transmits a radio frequency (RF) signal to the vehicle's body control module (BCM). Think of the BCM as the central nervous system of your Altima, managing various electrical functions. This signal contains encrypted information that identifies the fob as authorized. This prevents someone from simply spoofing the signal and starting your car. The specific frequency used varies depending on region, but it's typically within the 315 MHz or 433 MHz range.

Range is a critical factor. The effective range of the auto start is limited by the strength of the transmitter in the fob and the receiver in the BCM. Obstacles like walls and other electronic interference can significantly reduce the range. Keep this in mind when troubleshooting issues with starting from a distance.

BCM Verification and Immobilizer System

Upon receiving the RF signal, the BCM doesn't immediately start the engine. It first verifies the validity of the signal. This involves checking the encrypted data against a stored key code. If the key code matches and the signal is deemed legitimate, the BCM then communicates with the immobilizer system.

The immobilizer system is a security feature that prevents the engine from starting without the correct key. It typically involves a transponder chip embedded in the key fob. When the key is in the ignition (or in proximity with push-button start systems), the immobilizer verifies the transponder's code. In the case of remote start, the BCM temporarily bypasses this immobilizer check after verifying the RF signal. This is crucial because otherwise, the engine wouldn't start without the physical key being present.

Any fault in the immobilizer system will prevent the remote start from functioning, even if the key fob battery is good and the RF signal is reaching the BCM.

Safety Interlocks

Before engaging the starter motor, the BCM checks several safety interlocks. These are conditions that must be met to prevent accidental or unsafe operation:

Transmission in Park (P): The system must confirm that the transmission is in Park (for automatic transmissions) or Neutral (for manual transmissions – although the 2019 Altima doesn’t have a manual option). This is usually verified by a signal from the transmission range sensor.
Brake Pedal Not Depressed: The brake pedal must not be depressed. This prevents the car from accelerating unexpectedly during the remote start sequence.
Hood Closed: The hood must be closed. An open hood indicates that someone might be working on the engine, and starting it remotely could be dangerous.
No Diagnostic Trouble Codes (DTCs) related to Engine or Transmission: The engine control unit (ECU) or transmission control unit (TCU) cannot have any serious error codes that would prevent safe operation. The BCM often receives this information via the CAN bus (Controller Area Network), a communication network within the vehicle.

If any of these interlocks are not met, the auto start system will fail to activate. Common causes of auto start failure include a faulty hood latch sensor, a malfunctioning transmission range sensor, or an engine check light being illuminated.

Engine Starting Sequence

Once all interlocks are satisfied, the BCM signals the engine control unit (ECU) to begin the starting sequence. The ECU controls the fuel injection, ignition timing, and other critical engine functions. The ECU engages the starter motor, and the engine starts. The engine will typically run for a predetermined time, usually 10-15 minutes, unless manually shut off or the interlocks are violated during operation (e.g., someone opens the door). After the run time expires, the engine shuts off automatically.

Troubleshooting Common Issues

Here's a breakdown of common auto start problems and how to approach diagnosing them:

No Response from Key Fob:

Possible Causes: Dead key fob battery, key fob out of range, interference, faulty key fob, BCM receiver malfunction.

Troubleshooting Steps: Replace key fob battery, move closer to the vehicle, try a different location to rule out interference, try a different key fob (if available), check the BCM fuse.
Engine Cranks But Doesn't Start:

Possible Causes: Low fuel level, weak battery, faulty crankshaft position sensor, immobilizer system issue, DTCs related to engine or fuel system.

Troubleshooting Steps: Check fuel level, test battery voltage, scan for DTCs using an OBD-II scanner, check immobilizer system (may require professional diagnosis).
Engine Starts and Immediately Shuts Off:

Possible Causes: Safety interlock issue (e.g., hood open, transmission not in Park), security system malfunction, fuel delivery problem.

Troubleshooting Steps: Verify all safety interlocks are met, check the hood latch sensor, scan for DTCs related to security system or fuel system.
Reduced Range:

Possible Causes: Weak key fob battery, interference, corroded antenna connection for the BCM receiver.

Troubleshooting Steps: Replace key fob battery, try a different location, inspect the antenna connection to the BCM receiver (location varies by model year, consult a service manual).

Important Note: Before attempting any electrical repairs, disconnect the negative battery terminal. This will prevent accidental shorts and potential damage to the vehicle's electrical system.

Modifications and Aftermarket Systems

While Nissan's factory auto start system is generally reliable, some owners explore modifications or aftermarket upgrades. Reasons for this include extending the range, adding smartphone control, or integrating the system with other aftermarket security features.

When considering aftermarket systems, it's crucial to choose a reputable brand and have it professionally installed. Improper installation can lead to electrical problems, security vulnerabilities, or even damage to the vehicle's computer systems. Furthermore, modifying the factory auto start system could void your vehicle's warranty.

Bypassing the immobilizer is a common practice with some aftermarket remote start systems, but it introduces a significant security risk. Instead of a full bypass, look for systems that use a data interface module to communicate with the vehicle's immobilizer system in a secure and authorized manner.

Accessing Diagnostic Information

As mentioned earlier, using an OBD-II (On-Board Diagnostics II) scanner is essential for diagnosing auto start problems. A scanner can read DTCs stored in the ECU, BCM, and other modules, providing valuable clues about the cause of the issue. Some advanced scanners can even display live data from sensors related to the auto start system, such as the transmission range sensor and the hood latch sensor.

You'll also need access to the vehicle's wiring diagrams and service manuals. These resources provide detailed information about the location of components, wiring connections, and troubleshooting procedures. These manuals are often available online through subscription services or from reputable automotive parts suppliers.

Remember, working on automotive electrical systems requires a basic understanding of electrical principles, proper tools, and a cautious approach. If you're not comfortable with any of these aspects, it's best to seek professional assistance from a qualified mechanic.