P1790

What Does Code P1790 Mean?

The diagnostic trouble code P1790 indicates a malfunction within the Throttle Position (TP) mechanical circuit. This code is generated by the Engine Control Module (ECM) or Powertrain Control Module (PCM) when it detects an inconsistency or abnormal behavior in the throttle position sensor (TPS) signal that points towards a mechanical rather than a purely electrical fault. The ECM/PCM continuously monitors the TPS voltage output, which directly correlates with the physical angle of the throttle plate. A “mechanical circuit malfunction” implies that the system has identified an issue where the TPS signal does not smoothly or accurately reflect the actual throttle plate movement, or where the throttle plate itself is not moving as expected or commanded. This can manifest as an erratic, non-linear, or stuck reading from the TPS, even when the throttle plate should be moving or is commanded to move. The PCM uses this signal for critical engine functions such as fuel injection, ignition timing, and idle speed control. When this code sets, it signifies a subsystem failure affecting precise throttle control and potentially overall engine drivability and performance.

Common Symptoms

• Erratic or high idle speed: The engine may idle rough, fluctuate excessively, or idle at a higher RPM than normal.
• Stalling: The engine may stall, especially when coming to a stop or during deceleration.
• Hesitation or poor acceleration: A delay or lack of power when pressing the accelerator pedal.
• Unintended acceleration or deceleration: The engine RPMs may fluctuate without direct driver input.
• Lack of throttle response: The engine may not respond proportionally to accelerator pedal input.
• Cruise control inoperative: Cruise control systems often rely heavily on accurate throttle position data.
• Engine entering “limp mode”: The ECM/PCM may limit engine power and speed to prevent further damage.
• Check Engine Light (CEL) illuminated: The primary indicator that a fault has been detected.

What Causes the Code P1790?

• Faulty Throttle Position Sensor (TPS): Internal wear, degradation, or mechanical failure of the TPS potentiometer causing erratic or non-linear voltage output during throttle movement.
• Binding or sticking throttle plate/butterfly valve: Accumulation of carbon deposits, physical damage, or wear in the throttle body causing the plate to not move freely or smoothly.
• Excessive play or wear in the throttle shaft: Looseness in the shaft supporting the throttle plate can lead to inconsistent TPS readings.
• Malfunctioning electronic throttle control (ETC) motor or gears: For drive-by-wire systems, internal failure or wear within the throttle body’s electronic actuator mechanism, preventing smooth throttle plate operation.
• Damaged or misaligned throttle cable (for cable-actuated systems): A kinked, frayed, or improperly adjusted throttle cable can impede smooth throttle plate movement.
• Physical obstruction within the throttle body: Debris or foreign objects preventing the throttle plate from fully opening or closing.
• Bent or damaged throttle plate: Physical damage to the butterfly valve itself, causing it to bind within the throttle bore.

How to Diagnose and Troubleshoot

Diagnosing P1790 requires a methodical approach, often combining visual inspection with advanced diagnostic tools.

1. Visual Inspection:
   • Begin with a thorough visual inspection of the throttle body assembly. Look for obvious signs of damage, excessive carbon buildup around the throttle plate and bore, or any obstructions preventing smooth movement.
   • For cable-actuated systems, inspect the throttle cable for kinks, fraying, or improper tension/routing. Manually operate the throttle lever to ensure smooth, unhindered movement from fully closed to fully open positions. The throttle should snap back quickly and completely.
   • For drive-by-wire (ETC) systems, visually inspect the electrical connector to the throttle body for corrosion, damage, or loose pins. While not directly a mechanical issue, it’s good practice.
2. OBD-II Scanner Live Data Analysis:
   • Connect an OBD-II scanner capable of displaying live data. Monitor the Throttle Position Sensor (TPS) voltage or percentage (often labeled TPS1, TPS_A, or similar).
   • With the ignition ON and engine OFF (KOEO), slowly and steadily depress the accelerator pedal from idle to wide-open throttle (WOT). Observe the TPS readings. The voltage/percentage should increase smoothly and linearly without any sudden drops, spikes, or flat spots.
   • A typical idle voltage is around 0.5V-1.0V, increasing to approximately 4.0V-4.5V at WOT. Specific values vary by manufacturer.
   • If the vehicle has dual TPS sensors (TPS1 and TPS2), monitor both. They should often move in opposite directions or maintain a specific voltage ratio (e.g., TPS2 = 1/2 of TPS1). Inconsistencies between them can indicate a mechanical fault within the TPS or throttle body.
   • Observe the throttle plate’s physical movement during this process. If the plate sticks or moves erratically while the TPS reading is also erratic, it reinforces a mechanical issue.
3. Digital Multimeter (DMM) Testing (TPS out of circuit/back-probing):
   • TPS Reference Voltage: With the ignition ON, back-probe the TPS connector to verify the 5-volt reference (Vref) signal and a good ground connection. Low Vref or high resistance ground can mimic TPS issues.
   • TPS Signal Voltage: Back-probe the TPS signal wire. Slowly actuate the throttle by hand (engine OFF, ignition ON). The DMM voltage reading should smoothly sweep from idle voltage to WOT voltage. Any sudden drops to 0V, spikes, or plateaus indicate an internal mechanical fault within the TPS.
   • TPS Resistance Test (Sensor Disconnected): Disconnect the TPS connector. Using the ohmmeter function, measure resistance across the signal and reference/ground pins while manually actuating the throttle. The resistance should change smoothly and consistently. Erratic resistance changes or open circuits at specific points in the sweep confirm internal TPS failure.
4. Throttle Body Actuator Test (for ETC systems, if available):
   • If your scanner has bidirectional control, perform a throttle body actuator test. This allows you to command specific throttle plate positions. Observe if the throttle plate responds accurately and smoothly, and if the TPS readings correspond correctly.

Recommended Repairs and Solutions

Once the diagnostic steps have identified the specific mechanical fault, the following repairs are commonly indicated:

1. Throttle Position Sensor (TPS) Replacement: If DMM testing clearly points to an internal failure or erratic signal output from the sensor itself, replace the TPS. Ensure the new sensor is properly calibrated or adjusted according to manufacturer specifications (some TPS are not adjustable and are simply replaced).
2. Throttle Body Cleaning: If carbon buildup is causing the throttle plate to stick or bind, a thorough cleaning of the throttle body and butterfly plate is necessary. Use an appropriate throttle body cleaner and a soft brush, ensuring no cleaner residue contaminates the intake manifold. For ETC systems, be extremely cautious not to force the butterfly plate open, as this can damage the internal gearing or motor.
3. Throttle Body Replacement: If the throttle plate, shaft, or the electronic actuator mechanism (for drive-by-wire systems) is physically damaged, excessively worn, or cannot be repaired by cleaning, the entire throttle body assembly will need to be replaced. This is often the case when there’s significant play in the throttle shaft or internal ETC motor failure.
4. Throttle Cable Repair/Adjustment: For vehicles with a physical throttle cable, inspect and repair any damage. Adjust the cable tension to manufacturer specifications to ensure smooth, full range of motion without binding or excessive slack. Lubricate the cable if necessary.
5. ECM/PCM Reprogramming/Adaptation: After replacing a throttle body or TPS, it’s often crucial to perform an idle learn procedure or a throttle body adaptation using a capable scan tool. This allows the ECM/PCM to learn the new component’s minimum and maximum throttle positions and ensure proper idle and off-idle performance.

Mechanic’s Tip: Always clear the fault code after completing repairs and perform a drive cycle to confirm the issue is resolved and the code does not return. Pay close attention to any changes in throttle response or idle quality after the repair.