P1795 - CAN Bus - Throttle Body Position

Introduction:

The P1795 diagnostic trouble code (DTC) indicates an issue related to the throttle body position sensor (TPS) signal communicated over the Controller Area Network (CAN) bus. This code isn't directly about a faulty TPS itself, but rather a problem in how the Engine Control Unit (ECU) is receiving or interpreting the TPS data being transmitted across the CAN bus from another module, such as the Throttle Actuator Control (TAC) module or another computer on the vehicle. Understanding the CAN bus and its role in modern vehicles is crucial for diagnosing and resolving P1795.

Table: P1795 - CAN Bus - Throttle Body Position Details

Topic	Description	Possible Causes
Definition	CAN Bus - Throttle Body Position; Indicates a discrepancy or lack of communication regarding the throttle body position signal transmitted over the CAN bus. The ECU is not receiving valid TPS data via the CAN.	CAN bus wiring issues (shorts, opens, corrosion) Faulty module transmitting TPS data (e.g., TAC module) Faulty ECU (rare) CAN bus communication errors Software/firmware issues in modules communicating on the CAN bus Incorrect vehicle configuration/programming * Loose or corroded connectors in the CAN bus network
Symptoms	Check Engine Light (CEL) illuminated Reduced engine power (Limp mode) Erratic engine behavior Poor throttle response Stalling Difficulty starting * Transmission shifting problems (if TCM relies on TPS data)	N/A
CAN Bus Basics	Serial communication network allowing various vehicle modules (ECU, ABS, TCM, etc.) to communicate with each other without complex wiring harnesses. Data is transmitted in messages with identifiers indicating the message type and source.	CAN bus overload (too much data being transmitted) CAN bus collision (two modules trying to transmit simultaneously) * Incorrect termination resistors (120 ohms at each end of the bus)
Throttle Body Position Sensor (TPS)	Measures the throttle plate angle, providing the ECU with information about the driver's demand for power. Typically a potentiometer or Hall-effect sensor. P1795 usually isn't a direct fault of the TPS itself, but a communication issue related to its signal.	Rarely, a faulty TPS could* cause a module to send incorrect data over the CAN bus, triggering P1795.
Throttle Actuator Control (TAC) Module	Controls the throttle plate opening and closing based on driver input and ECU commands. Often integrates the TPS. Responsible for sending the TPS signal over the CAN bus in many vehicles.	TAC module failure Wiring issues between the TPS and the TAC module
Diagnostic Steps	1. Scan for other DTCs (CAN bus related codes, TPS codes). 2. Inspect CAN bus wiring and connectors for damage, corrosion. 3. Check CAN bus termination resistors. 4. Monitor TPS data on a scan tool (if possible) for inconsistencies. 5. Check for TSBs related to P1795 and the specific vehicle. 6. Test the TAC module (if equipped) using a multimeter and scan tool. 7. Check for software updates for relevant modules.	N/A
Related Codes	P0120-P0125 (TPS circuit malfunctions), U-codes (CAN bus communication errors), P2100-P2119 (Throttle Actuator Control System).	N/A
Tools Required	OBD-II scanner with CAN bus diagnostic capabilities Multimeter Wiring diagrams for the specific vehicle Scan tool with live data monitoring capabilities * CAN bus analyzer (advanced troubleshooting)	N/A
Severity	Moderate to High. Can result in reduced engine power, stalling, and potentially unsafe driving conditions.	N/A
Repair Costs	Varies widely. Could range from a simple wiring repair (under $100) to replacing a module like the TAC module or ECU (several hundred to over a thousand dollars). Diagnosis time can also significantly impact costs.	N/A

Detailed Explanations:

Definition:

The P1795 code signifies that the ECU isn't receiving a valid or consistent throttle body position signal via the CAN bus. This doesn't necessarily mean the TPS itself is faulty, but rather the communication pathway conveying the TPS data is compromised. The ECU relies on accurate TPS data to control fuel delivery, ignition timing, and other crucial engine functions, so a communication breakdown leads to the code being set.

Symptoms:

The symptoms of P1795 can vary depending on the severity of the communication issue and the specific vehicle's design. Common symptoms include the Check Engine Light illuminating, reduced engine power (often referred to as "limp mode"), erratic engine behavior, poor throttle response, stalling, and difficulty starting. In some cases, transmission shifting problems might also occur because the Transmission Control Module (TCM) relies on TPS data for proper shift timing.

CAN Bus Basics:

The CAN bus is a crucial communication network in modern vehicles. It allows various electronic control modules (ECMs) like the ECU, ABS, TCM, and others to exchange information efficiently. Instead of each module having its own dedicated wiring harness to every other module, the CAN bus provides a shared communication pathway. Data is transmitted in messages, each with a unique identifier indicating the message type and the module that sent it. This reduces wiring complexity and improves reliability. CAN bus communication relies on a twisted pair of wires (CAN High and CAN Low) and requires termination resistors at each end of the bus to prevent signal reflections.

Throttle Body Position Sensor (TPS):

The Throttle Body Position Sensor (TPS) is a critical component that measures the angle of the throttle plate. This information is relayed to the ECU, which uses it to determine the driver's demand for power and adjust engine parameters accordingly. The TPS is typically a potentiometer or a Hall-effect sensor that produces a voltage signal proportional to the throttle plate angle. It's important to emphasize that P1795 usually isn't directly caused by a faulty TPS itself. The issue lies in the communication of the TPS signal over the CAN bus. However, a faulty TPS could, in rare cases, send erroneous data, indirectly triggering the P1795 code.

Throttle Actuator Control (TAC) Module:

The Throttle Actuator Control (TAC) module is responsible for controlling the opening and closing of the throttle plate based on driver input and commands from the ECU. In many vehicles, the TAC module integrates the TPS and is responsible for transmitting the TPS signal over the CAN bus to the ECU. If the TAC module fails or has internal communication problems, it can lead to the P1795 code. Wiring issues between the TPS and the TAC module can also cause this problem.

Diagnostic Steps:

Diagnosing P1795 requires a systematic approach:

Scan for other DTCs: Use an OBD-II scanner to check for any other diagnostic trouble codes, especially CAN bus-related codes (U-codes) or TPS-related codes (P0120-P0125). These codes can provide valuable clues about the underlying issue.
Inspect CAN bus wiring and connectors: Carefully inspect the CAN bus wiring and connectors for any signs of damage, corrosion, or loose connections. Pay close attention to the wiring near the ECU, TAC module, and any other modules connected to the CAN bus.
Check CAN bus termination resistors: The CAN bus requires termination resistors (typically 120 ohms) at each end of the bus to prevent signal reflections. Use a multimeter to measure the resistance between the CAN High and CAN Low wires at the DLC (Data Link Connector - OBD-II port) with the ignition off. A reading of approximately 60 ohms indicates that the termination resistors are present and functioning correctly.
Monitor TPS data on a scan tool: If possible, use a scan tool with live data monitoring capabilities to monitor the TPS data being transmitted over the CAN bus. Look for any inconsistencies, dropouts, or erratic readings. Compare the TPS data with the actual throttle plate position.
Check for TSBs: Check for any Technical Service Bulletins (TSBs) related to P1795 and the specific vehicle model. TSBs often provide valuable information about common problems and recommended solutions.
Test the TAC module (if equipped): If the vehicle is equipped with a TAC module, test it using a multimeter and a scan tool. Follow the manufacturer's diagnostic procedures for testing the TAC module.
Check for software updates: Check for any available software updates for the ECU, TAC module, or other modules communicating on the CAN bus. Software glitches can sometimes cause communication problems.

Related Codes:

Several other DTCs can be related to P1795, including:

P0120-P0125: TPS circuit malfunctions (these codes may indicate a problem with the TPS itself, which could indirectly cause P1795).
U-codes: CAN bus communication errors (these codes indicate general problems with the CAN bus network).
P2100-P2119: Throttle Actuator Control System (these codes indicate problems with the TAC system).

Tools Required:

Diagnosing P1795 requires several specialized tools:

OBD-II scanner with CAN bus diagnostic capabilities: This is essential for reading and clearing DTCs, as well as monitoring live data.
Multimeter: Used for checking wiring continuity, voltage, and resistance.
Wiring diagrams for the specific vehicle: These are crucial for identifying the CAN bus wiring and components.
Scan tool with live data monitoring capabilities: Allows you to monitor TPS data and other relevant parameters in real-time.
CAN bus analyzer (advanced troubleshooting): A more advanced tool used for analyzing CAN bus traffic and identifying communication problems. This is typically used by experienced technicians.

Severity:

The severity of P1795 can range from moderate to high. In some cases, it may only cause a minor inconvenience, such as a Check Engine Light and slightly reduced engine power. However, in more severe cases, it can lead to stalling, poor throttle response, and potentially unsafe driving conditions.

Repair Costs:

The cost of repairing P1795 can vary widely depending on the cause of the problem. A simple wiring repair might cost under $100, while replacing a module like the TAC module or ECU could cost several hundred to over a thousand dollars. Diagnosis time can also significantly impact the overall cost.

Frequently Asked Questions:

What does P1795 mean? It means the ECU isn't receiving valid throttle body position data over the CAN bus. This indicates a communication problem, not necessarily a faulty TPS.
Is P1795 a serious problem? Yes, it can be. It can lead to reduced engine power, stalling, and unsafe driving conditions.
Can I fix P1795 myself? It depends on your mechanical skills and diagnostic abilities. Start with a visual inspection of wiring and connectors. More complex diagnostics require specialized tools.
Will replacing the TPS fix P1795? Not usually. P1795 indicates a CAN bus communication problem, not a direct TPS failure, but it's not entirely impossible.
What is the CAN bus? It's a communication network that allows different electronic modules in your car to "talk" to each other. It simplifies wiring and improves reliability.

Conclusion:

The P1795 DTC points to a communication problem on the CAN bus related to the throttle body position signal. Thorough diagnosis, including checking wiring, CAN bus integrity, and the functionality of relevant modules like the TAC module, is crucial for resolving this issue. Addressing the root cause promptly will ensure optimal engine performance and safe driving conditions.