P0333 on 2019-2020 GMC Sierra 2500: Knock Sensor Circuit Causes and Fixes

What's Unique About the 2019-2020 Gmc SIERRA 2500

For this specific truck and its platform mates, a P0333 code has a well-documented cause that is not a simple sensor failure. General Motors issued Technical Service Bulletin #20-NA-207, which states that an electrical issue at the ECM's X2 connector can trigger this code. Voltage from the transmission's park/neutral signal circuit can sometimes 'bleed' into the knock sensor circuit within the connector, creating the false high voltage reading. This makes inspecting the ECM connector, specifically looking for debris or moisture at Pin 52 (Knock Sensor 2 Signal, typically a Dark Blue wire), the most critical first step ahead of replacing any parts.

Diagnostic Flowchart

Tap your situation to follow the diagnostic path that matches what you're seeing on this vehicle. 👇

Generation note: The 2019-2020 range covers the end of the 3rd generation (K2XX platform, 2019) and the start of the 4th generation (T1XX platform, 2020). The provided TSB #20-NA-207 was updated to include 2019-2020 models, indicating the electrical issue at the ECM connector can affect both generations.

Symptoms You May Notice

• Check Engine Light (Malfunction Indicator Lamp) is on
• Reduced engine power and sluggish acceleration.
• Check Engine Light may appear specifically when shifting out of Park.
• Audible engine pinging or knocking (less common)
• Decreased fuel economy.
• Rough or uneven idle.

Most Likely Causes

1. Electrical Issue at ECM X2 Connector 🔴 High Probability As documented in GM TSB #20-NA-207, these vehicles are susceptible to moisture, debris, or poor terminal contact in the ECM's X2 connector, which can cause voltage from the park/neutral signal circuit to interfere with the knock sensor circuit (Pin 52).
   How to confirm: Disconnect the battery, then locate and disconnect the K20 ECM's X2 connector. Visually inspect for any debris, moisture, or corroded/damaged terminals, paying close attention to the area around Pin 52. The code setting immediately after shifting from Park is a strong indicator of this issue.
   Typical fix: Clean the connector thoroughly with electrical contact cleaner, let it dry completely, and apply a small amount of dielectric grease to the connector seal to prevent future moisture intrusion. Ensure a secure connection. If terminals are damaged, they may need to be professionally repaired or replaced.
   Est. part cost: $5-$25
2. Faulty Knock Sensor (Bank 2) 🟡 Medium Probability → Shop Ignition Knock (Detonation) Sensor The sensor is a piezoelectric crystal that can fail over time due to extreme heat cycles and constant engine vibration. Its location on the passenger side of the engine block exposes it to road debris and moisture, which can lead to failure of the sensor or its pigtail connector.
   How to confirm: If the ECM connector is clean and the code returns, test the sensor's resistance with a multimeter. It should be within the manufacturer's specification (typically 93-110 kOhms). An open (infinite resistance) or shorted (near-zero resistance) reading indicates a failure. You can also check for AC voltage from the sensor while tapping on the engine block near it (engine off). 🎬 See this guide on how to diagnose GM knock sensors.
   Typical fix: Replace the Bank 2 (passenger side) knock sensor. This often requires removing the starter motor for access. 🎬 Watch: A pro walkthrough on replacing the knock sensor. Ensure the new sensor is torqued to specification, which is critical for proper function (usually 18 ft-lbs).
   Est. part cost: $30-$90
3. Damaged Wiring Harness ⚪ Low Probability Engine heat can make wires brittle, and vibrations can cause the harness to chafe against engine components, brackets, or the firewall, leading to an open circuit or a short to voltage. GM has issued other TSBs (e.g., #21-NA-149) regarding general harness chafing issues on these platforms.
   How to confirm: Visually inspect the wiring harness from the Bank 2 knock sensor to the ECM. Check for any melted, chafed, or broken wires. Perform a continuity test on the signal wire (typically Dark Blue at ECM X2, Pin 52) and check for a short to voltage or ground.
   Typical fix: Repair the damaged section of the wire with a weatherproof splice or replace the engine wiring harness if damage is extensive. In some cases, only the sensor pigtail connector needs replacement.
   Est. part cost: $10-$500+

Rare But Worth Checking

• Faulty Engine Control Module (ECM): → Shop Engine Control Module (ECM) This is extremely rare. The ECM should only be considered after all other possibilities, including the TSB-related connector issue, the sensor itself, and the entire wiring harness, have been exhaustively ruled out.

Diagnosis Steps

1. Read the trouble codes with an OBD-II scanner to confirm P0333 is the primary code.
2. IMPORTANT: Following GM TSB #20-NA-207, disconnect the negative battery terminal.
3. Locate and inspect the K20 ECM X2 electrical connector (often the gray or black connector). Look for debris, moisture, backed-out pins, or corrosion, especially around Pin 52.
4. If any issues are found, clean the connector with electrical contact cleaner, allow it to dry, apply dielectric grease to the seal, and ensure a secure, latched connection.
5. Clear the codes, reconnect the battery, and drive the vehicle. Specifically test by shifting from Park to Drive to see if the code returns. If it does not, the problem is solved.
6. If the code returns, locate the Bank 2 (passenger's side) knock sensor on the engine block. Access may require removing the starter motor.
7. Visually inspect the sensor and its pigtail connector for damage, corrosion, or melting.
8. Test the knock sensor's internal resistance with a multimeter. Compare the reading to the factory specification (e.g., 93-110 kOhms). An out-of-spec reading confirms a bad sensor.
9. If the sensor tests bad, replace it and torque the new sensor's mounting bolt to 18 ft-lbs.
10. If the sensor tests good, perform a continuity test on the signal wire (Dark Blue, Pin 52) from the sensor connector to the ECM connector. Also check for shorts to voltage and ground.

Parts You'll Likely Need

• Knock Sensor (OEM #12623730) — This is the sensor that reports the data for the P0333 code. It is the second most likely cause of failure after the ECM connector issue. This part number fits a wide range of GM V8 engines.
  Trusted brands: ACDelco, GM Genuine, Delphi
  OEM price range: $50-$90
  Aftermarket price range: $30-$60
• Electrical Contact Cleaner — Required for cleaning the ECM X2 connector, which is the most likely cause of the problem per TSB #20-NA-207.
  Trusted brands: CRC, WD-40 Specialist
  Aftermarket price range: $5-$15
• Dielectric Grease — Recommended to apply to the ECM connector seal after cleaning to prevent future moisture intrusion and corrosion.
  Trusted brands: Permatex, CRC
  Aftermarket price range: $5-$10

Technical Service Bulletins (TSBs) & Recalls

• 20-NA-207: Malfunction Indicator Lamp (MIL) Illuminated - DTC P0333 Set. Points to a possible electrical concern at the ECM X2 electrical connector. This bulletin supersedes PIP5576.
• PIP5576: The preliminary information bulletin that was the precursor to 20-NA-207, indicating a history of this known issue.

Platform-Specific Known Issues

• TSB #20-NA-207: GM has identified that a Malfunction Indicator Lamp (MIL) may illuminate, setting DTC P0333, due to an electrical concern at the ECM X2 electrical connector. The bulletin notes this can happen when shifting from Park as voltage from the transmission park/neutral signal circuit bleeds into the knock sensor circuit.
• ECM Connector Pinpoint: The specific circuit identified in diagnostics is the Knock Sensor 2 signal wire, which is located at Pin 52 of the X2 connector on the K20 ECM.

Mechanic-Grade Diagnostic Values

• Knock Sensor Internal Resistance — expected: 93,000 to 110,000 ohms (93k-110k Ω). Failure: A multimeter reading of OL (open loop/infinite resistance) or a value significantly outside the expected range.
• Knock Sensor AC Voltage Output (Dynamic Tap Test) — expected: A small AC voltage signal, typically greater than 50mV to 1V, generated when tapping the engine block near the sensor with the engine off.. Failure: Very low or no AC voltage (e.g., less than 10 millivolts) produced when tapping, suggesting the sensor's piezoelectric element is dead.
• Knock Sensor Circuit Voltage (Key On, Engine Off) — expected: Approximately 5 Volts DC (bias voltage supplied by the ECM).. Failure: A P0333 'Circuit High' code is set when the voltage reading is stuck high (often above 4.5V) and is not being pulled down by the sensor's internal resistor.
• Knock Sensor Mounting Bolt Torque — expected: 15-18 ft-lbs (20-25 Nm). Failure: This is an installation specification, not a diagnostic value. However, incorrect torque (either too loose or too tight) can alter the sensor's sensitivity and lead to false readings or failure to detect real knock.

Scan Tool Commands That Help

• GDS2 (GM Global Diagnostic System 2): Engine Data Display - Live Data — Use this to monitor the voltage from both Bank 1 and Bank 2 knock sensors simultaneously. For P0333, a technician would look for the Bank 2 voltage to be pegged high or show erratic readings compared to the healthy Bank 1 sensor, especially when shifting out of Park as described in TSB #20-NA-207.
• GDS2 / Professional Scan Tool: Knock Retard / Knock Sensor Active Counter — During a test drive, monitoring these parameters can show if the ECM is actively retarding timing. With a P0333 fault, you will likely see zero knock retard activity for Bank 2, as the ECM has invalidated the signal and is ignoring it.

Wiring & Ground Locations

• K20 Engine Control Module (ECM) — For 2019 (K2XX) models, it's typically on the passenger side of the engine compartment, near the firewall. For 2020 (T1XX) models, it is located in the front driver's side wheel well, behind the felt liner.. This is the location of the X2 connector, which is the primary suspect for the P0333 code according to TSB #20-NA-207. Physical access is required for inspection and cleaning.
• ECM X2 Connector, Pin 52 — This is the specific pin for the Knock Sensor 2 Signal circuit within the X2 connector at the ECM. The wire is typically Dark Blue.. TSB #20-NA-207 points to issues like moisture or debris at this specific connector causing voltage bleed-over, making this the exact pin to inspect and test for continuity and shorts.
• G103 / G104 — These are common engine block ground points, often located on the rear of the cylinder heads (one on each side). For example, on some GM V8s, G103 is at the left front of the engine block.. While not specifically mentioned in the P0333 TSB, a loose, corroded, or broken main engine ground can cause a wide range of phantom electrical issues and incorrect sensor readings. Verifying these grounds are clean and tight is a crucial step in any electrical diagnosis.

Real Owner Repair Stories

• gm-trucks.com forum user (2015 GMC Sierra 5.3L (K2XX Platform, same as 2019)) — Intermittent Check Engine Light with P0333.
  ❌ Tried (didn't work) Replaced the Bank 2 knock sensor twice.
  ✅ What actually fixed it The owner found TSB #PIP5576 (the precursor to #20-NA-207), disconnected the ECM connectors, cleaned them with electrical contact cleaner, and applied dielectric grease. The code did not return, confirming the fault was in the connector, not the sensor.

OEM Part Supersession History

• 12570125 → 12623730 — Standard part update/revision by GM.
  Heads up: The new part number (12623730) is the correct replacement for the old one.
• 12623730 → 12739206 — Standard part update/revision by GM.
  Heads up: Some parts catalogs show 12739206 as a further replacement for 12623730. When ordering, it is best to confirm fitment with the vehicle's VIN.

Model Year Variations Within This Range

• 2019 vs 2020: The physical location of the Engine Control Module (ECM) changed between the 2019 (K2XX platform) and 2020 (T1XX platform) models. On the 2019, it is generally in the engine bay on the passenger side firewall. On the 2020, it was moved to the front driver's side wheel well, behind the fender liner. This significantly changes the procedure for accessing the X2 connector mentioned in the TSB.