Introduction:

The diagnostic trouble code (DTC) P1891 indicates a problem within the throttle position sensor (TPS) circuit, specifically relating to a low pulse width modulation (PWM) signal. This code is most commonly associated with Ford vehicles, although similar codes may appear on other makes and models with variations in the description. Understanding the causes, symptoms, and diagnostic procedures for P1891 is crucial for accurate diagnosis and effective repair, ensuring optimal engine performance and drivability.

Topic Description Possible Causes
Definition Throttle Position Sensor (TPS) Pulse Width Modulation (PWM) Signal Low. Indicates that the powertrain control module (PCM) has detected a PWM signal from the TPS that is lower than expected or outside of the acceptable range. This signal is critical for the PCM to accurately determine throttle position. Short to ground in the TPS wiring, faulty TPS sensor, PCM malfunction (rare), poor connection at the TPS connector, wiring harness damage, corrosion in the connectors, incorrect TPS adjustment (if adjustable), low voltage to the TPS.
Symptoms Reduced engine power, poor acceleration, stalling, hesitation, erratic idling, transmission shifting problems (hard shifts, delayed shifts), Check Engine Light illuminated, vehicle may enter "limp mode". Faulty TPS sensor, wiring issues (short to ground, open circuit), PCM problems (less common), vacuum leaks (affecting idle), throttle body issues (sticking, carbon buildup).
TPS Function The TPS measures the throttle plate's angle and sends this information to the PCM as a voltage or PWM signal. The PCM uses this data to calculate fuel delivery, ignition timing, and transmission shift points. The PWM signal is a square wave where the 'on' time (pulse width) represents the throttle position. Failure of the TPS sensor, mechanical binding of the throttle plate, degradation of the TPS signal due to age or contamination.
PWM Signal Pulse Width Modulation is a technique used to represent analog values with a digital signal. The width of the pulse corresponds to the value being represented. In the case of the TPS, a wider pulse indicates a larger throttle opening. Interference in the signal, short to ground pulling down the pulse width, open circuit preventing the signal from reaching the PCM at the correct amplitude, incorrect signal frequency.
Diagnostic Tools Multimeter, scan tool with live data capabilities, oscilloscope (optional, for detailed PWM signal analysis), wiring diagrams. Lack of proper diagnostic tools, incorrect tool usage, misinterpretation of scan tool data, failure to perform a thorough visual inspection.
Troubleshooting Steps Visual inspection of wiring and connectors, testing TPS sensor voltage and resistance, checking for shorts to ground, verifying PCM ground and power, monitoring TPS data with a scan tool, performing a throttle body cleaning, replacing the TPS sensor (after confirming other issues are ruled out). Skipping steps in the diagnostic process, focusing solely on the TPS sensor without checking the wiring, failing to properly test the sensor, using incorrect testing procedures, replacing the TPS sensor without a confirmed diagnosis.
Related Codes P0120, P0121, P0122, P0123, P0124, P0220, P0221, P0222, P0223, P0224. These codes often indicate related issues with the TPS circuit or throttle body performance. Ignoring related codes, misinterpreting the relationship between codes, failing to address multiple issues simultaneously.
Potential Fixes Repairing or replacing damaged wiring, cleaning or replacing corroded connectors, replacing the TPS sensor, cleaning the throttle body, adjusting the TPS sensor (if adjustable), replacing the PCM (rare). Incorrect repair procedures, using low-quality replacement parts, failing to properly diagnose the root cause of the problem, neglecting to address underlying issues such as wiring damage or corrosion.
Safety Precautions Disconnect the negative battery terminal before working on electrical components. Be careful when working around the throttle body, as it can be sharp. Use proper safety glasses and gloves. Ignoring safety precautions, working on electrical components with the battery connected, failing to use proper safety equipment.
PCM Role The PCM (Powertrain Control Module) receives the TPS signal, interprets it, and uses it to control various engine functions, including fuel injection, ignition timing, and transmission shifting. A faulty PCM can misinterpret a good signal or fail to recognize a valid TPS signal. Internal PCM failure, damage to PCM connectors or wiring, incorrect PCM programming.

Detailed Explanations:

Definition:

The DTC P1891, "Throttle Position Sensor (TPS) Pulse Width Modulation (PWM) Signal Low," signifies that the PCM is receiving a PWM signal from the TPS that is lower than the expected range for a given throttle position. The PCM monitors this signal to control engine functions, and a low signal indicates a potential problem that needs to be addressed. This code is often seen in Ford vehicles but can appear in other makes with similar systems.

Symptoms:

The symptoms associated with a P1891 code can vary depending on the severity of the problem. Common symptoms include reduced engine power, poor acceleration, stalling, hesitation, and erratic idling. You might also experience transmission shifting problems such as hard or delayed shifts. The Check Engine Light will be illuminated, and in some cases, the vehicle may enter "limp mode," restricting engine performance to prevent further damage.

TPS Function:

The Throttle Position Sensor (TPS) is a crucial component that monitors the position of the throttle plate within the throttle body. As the driver presses the accelerator pedal, the throttle plate opens, and the TPS sends a corresponding signal to the PCM. This signal, either a voltage or PWM signal, indicates the degree of throttle opening, allowing the PCM to adjust fuel delivery, ignition timing, and transmission shift points accordingly for optimal engine performance.

PWM Signal:

Pulse Width Modulation (PWM) is a digital signaling technique used to represent analog values. In the context of the TPS, the width of the pulse in the PWM signal corresponds to the throttle position. A wider pulse signifies a larger throttle opening, while a narrower pulse indicates a smaller throttle opening. The PCM interprets this pulse width to determine the driver's demand for power.

Diagnostic Tools:

Diagnosing a P1891 code requires specific tools. A multimeter is essential for checking voltage, resistance, and continuity in the TPS circuit. A scan tool with live data capabilities allows you to monitor the TPS signal in real-time and observe its behavior as the throttle is moved. An oscilloscope (optional) can provide a more detailed analysis of the PWM signal waveform. Finally, wiring diagrams are crucial for understanding the circuit layout and identifying potential fault locations.

Troubleshooting Steps:

  1. Visual Inspection: Begin by thoroughly inspecting the TPS wiring and connectors for any signs of damage, corrosion, or loose connections.
  2. Voltage and Resistance Testing: Use a multimeter to test the TPS sensor's voltage and resistance, comparing your readings to the manufacturer's specifications.
  3. Short to Ground Check: Check for shorts to ground in the TPS wiring using a multimeter.
  4. PCM Ground and Power Verification: Ensure that the PCM is receiving proper ground and power.
  5. Scan Tool Data Monitoring: Monitor the TPS data with a scan tool while moving the throttle to observe any irregularities in the signal.
  6. Throttle Body Cleaning: Clean the throttle body to remove any carbon buildup that may be affecting throttle plate movement.
  7. TPS Sensor Replacement: If all other tests fail, consider replacing the TPS sensor, but only after confirming that other issues have been ruled out.

Related Codes:

Several other diagnostic trouble codes can be related to P1891, often indicating issues within the TPS circuit or throttle body performance. These include:

  • P0120: Throttle/Pedal Position Sensor/Switch A Circuit
  • P0121: Throttle/Pedal Position Sensor/Switch A Circuit Range/Performance
  • P0122: Throttle/Pedal Position Sensor/Switch A Circuit Low Input
  • P0123: Throttle/Pedal Position Sensor/Switch A Circuit High Input
  • P0124: Throttle/Pedal Position Sensor/Switch A Circuit Intermittent
  • P0220: Throttle/Pedal Position Sensor/Switch B Circuit
  • P0221: Throttle/Pedal Position Sensor/Switch B Circuit Range/Performance
  • P0222: Throttle/Pedal Position Sensor/Switch B Circuit Low Input
  • P0223: Throttle/Pedal Position Sensor/Switch B Circuit High Input
  • P0224: Throttle/Pedal Position Sensor/Switch B Circuit Intermittent

Addressing these related codes can often help resolve the underlying cause of the P1891 code.

Potential Fixes:

Based on the diagnostic findings, potential fixes for a P1891 code include:

  • Repairing or replacing damaged wiring in the TPS circuit.
  • Cleaning or replacing corroded connectors to ensure proper signal transmission.
  • Replacing the TPS sensor if it is found to be faulty.
  • Cleaning the throttle body to remove carbon buildup.
  • Adjusting the TPS sensor (if adjustable) to the correct position.
  • Replacing the PCM (rare), only if all other possibilities have been exhausted and the PCM is confirmed to be the source of the problem.

Safety Precautions:

Always prioritize safety when working on automotive electrical systems. Disconnect the negative battery terminal before beginning any work to prevent electrical shocks or damage to the vehicle's electronics. Be careful when working around the throttle body, as it can have sharp edges. Wear safety glasses and gloves to protect yourself from potential hazards.

PCM Role:

The Powertrain Control Module (PCM) is the central computer in the vehicle, responsible for controlling various engine functions. The PCM receives the TPS signal, interprets it, and uses this information to adjust fuel injection, ignition timing, and transmission shifting. A faulty PCM can misinterpret a good TPS signal or fail to recognize a valid signal altogether, leading to the P1891 code. While less common, PCM failure should be considered as a potential cause, especially if other diagnostic tests have not yielded a solution.

Frequently Asked Questions:

  • What does the P1891 code mean? The P1891 code indicates that the PCM has detected a low PWM signal from the Throttle Position Sensor, suggesting a problem within the TPS circuit.

  • Can a bad TPS cause a car to stall? Yes, a faulty TPS can cause a car to stall, especially at idle or during deceleration, due to the incorrect throttle position information being sent to the PCM.

  • How do I test a TPS sensor? You can test a TPS sensor using a multimeter to check its voltage and resistance across its terminals, comparing the readings to the manufacturer's specifications.

  • Is it safe to drive with a P1891 code? Driving with a P1891 code is not recommended, as it can lead to reduced engine performance, stalling, and potential damage to the engine or transmission.

  • Can a dirty throttle body cause a P1891 code? Yes, a dirty throttle body can cause the throttle plate to stick, affecting the TPS signal and potentially triggering a P1891 code.

Conclusion:

The P1891 code indicates a problem with the TPS PWM signal, often leading to drivability issues. A thorough diagnostic process, including visual inspection, component testing, and data analysis, is essential to pinpoint the root cause and implement the correct repair, ensuring optimal engine performance and reliability.