P1204

What Does Code P1204 Mean?

The diagnostic trouble code P1204 indicates an electrical malfunction within the fuel injector circuit for cylinder number 4. Specifically, the Engine Control Module (ECM), also known as the Powertrain Control Module (PCM), has detected either an “open circuit” or a “short circuit” condition. An open circuit implies a break in the electrical path, preventing current flow, while a short circuit signifies an unintended path for current flow, often to ground or a voltage source, leading to excessive current or voltage drop. The ECM monitors the electrical characteristics (resistance, voltage drop, current draw) of each injector’s driver circuit. When the measured parameters for cylinder #4 fall outside the manufacturer’s specified operational range during an injector pulse command, the ECM registers this anomaly and sets the P1204 code. This directly impairs the ECM’s ability to precisely control fuel delivery to cylinder #4, leading to incorrect air-fuel mixture and potential misfire conditions in that cylinder.

Common Symptoms

• Engine Misfire: Noticeable rough running, especially at idle or under load, often accompanied by a cylinder 4 specific misfire code (P0304).
• Reduced Engine Performance: Loss of power, sluggish acceleration, and overall poor drivability.
• Check Engine Light (CEL) Illumination: The Malfunction Indicator Lamp (MIL) will be activated on the dashboard.
• Rough Idle and Stalling: The engine may idle erratically, shake, or stall, particularly when cold or coming to a stop.
• Increased Fuel Consumption: In some cases, if the ECM attempts to compensate or if fuel is not efficiently burned, fuel economy may decrease.
• Unusual Odors: A strong smell of unburnt fuel from the exhaust if the injector is failing to open or close correctly, or if fuel is not being ignited.

What Causes the Code P1204?

• Faulty Fuel Injector #4: The most common cause, where the internal coil winding of the injector itself has failed, leading to an open circuit (infinite resistance) or a short circuit (near zero resistance or short to the injector body).
• Damaged Wiring Harness: An open circuit (broken wire) or a short circuit (wire chafing and contacting ground or another wire/voltage source) in the wiring leading to fuel injector #4 from the ECM/PCM. This can be due to heat, vibration, rodent damage, or improper installation.
• Corroded or Loose Electrical Connector: Poor electrical contact at the fuel injector #4 connector or at the corresponding ECM/PCM connector pins. This can be caused by moisture intrusion, bent pins, or insufficient terminal tension.
• Faulty Engine Control Module (ECM/PCM): Although less common, an internal failure of the injector driver circuit within the ECM responsible for cylinder #4 can cause this code.

How to Diagnose and Troubleshoot

Diagnosis of P1204 requires a systematic approach, utilizing a digital multimeter (DMM) and an OBD-II scan tool:

1. Verify Code and Related DTCs: Use an OBD-II scan tool to confirm P1204 is present. Check for any accompanying misfire codes (e.g., P0304) or other injector-related codes, which can provide additional context.
2. Visual Inspection:
   • Carefully inspect the wiring harness for cylinder #4 fuel injector. Look for any signs of physical damage, such as chafing, cuts, pinches, burn marks, or corrosion.
   • Examine the electrical connector at injector #4. Check for bent pins, corrosion, loose connections, or melted plastic. Ensure the connector is securely seated.
   • Follow the injector harness back towards the ECM/PCM, inspecting for any potential points of damage.
3. Electrical Resistance Test (Ignition OFF):
   • Disconnect the electrical connector from fuel injector #4.
   • Using a DMM set to ohms, measure the resistance across the two terminals of the injector itself.
   • Compare this reading to manufacturer specifications (typically 10-16 ohms for high-impedance injectors or 2-5 ohms for low-impedance injectors). An “OL” (open line) reading indicates an open circuit, while a reading near 0 ohms indicates a short circuit within the injector coil. If the reading is outside specification, the injector is faulty.
4. Wiring Harness Continuity and Short Tests (Ignition OFF, ECM/PCM Disconnected):
   • Locate and disconnect the main ECM/PCM connector(s) that carry the injector #4 circuit wires. This is crucial to prevent damage to the ECM/PCM during testing.
   • Identify the two wires for injector #4 at both the injector connector and the ECM/PCM connector.
   • Continuity Test: Using a DMM, check for continuity (ohms) between the corresponding pins at the injector connector and the ECM/PCM connector for each wire. Readings should be very low (typically < 5 ohms). An “OL” reading indicates an open circuit in the wire.
   • Short to Ground Test: With one DMM lead connected to a known good chassis ground (e.g., battery negative terminal), touch the other lead to each of the injector #4 circuit wires at the injector connector (or the ECM/PCM end). Readings should be “OL”. Any low resistance indicates a short to ground.
   • Short to Voltage Test: With the ignition ON (but ECM/PCM still disconnected and injector connector disconnected), check for voltage at each of the injector #4 circuit wires at the injector connector. There should be no voltage present on the ECM-controlled ground-side wire. The power supply wire should show battery voltage. If the ground-side wire shows voltage, there’s a short to voltage.
5. Power Supply Test (Ignition ON, Injector Disconnected):
   • With the injector #4 connector disconnected, turn the ignition to the ON position.
   • Using a DMM set to volts, check for battery voltage (B+) at one of the two pins within the injector #4 harness connector. This is the constant power supply, typically originating from a fuel injector relay or main power relay. The other pin will be the ECM’s ground-side driver. If no voltage is present, troubleshoot the power supply circuit (fuse, relay, wiring).
6. Injector Swap Test (If Applicable): If you suspect the injector itself but are unsure, and if other injectors are identical and easily accessible, swap injector #4 with a known good injector from another cylinder. Clear codes and re-test. If the code follows the injector to the new cylinder (e.g., P1203 appears for cylinder #3), then the injector is faulty. If P1204 remains, the issue is with the wiring or ECM/PCM.

Recommended Repairs and Solutions

Based on the diagnostic findings, apply the appropriate repair:

1. Replace Faulty Fuel Injector #4: If the resistance test of the injector confirmed an open or short circuit, replace the injector. Always use a quality OEM or equivalent replacement part. Ensure proper installation with new O-rings and seals to prevent fuel leaks.
2. Repair or Replace Wiring Harness: If diagnostic tests revealed an open or short in the wiring harness, repair the damaged section using automotive-grade solder, heat-shrink tubing, and appropriate wire gauge. For extensive damage or multiple breaks, replacement of the entire sub-harness may be more practical and reliable. Ensure proper routing and securing of the wiring to prevent future damage.
3. Clean or Replace Electrical Connectors: If corrosion, bent pins, or poor terminal tension was identified, carefully clean the connector pins using electrical contact cleaner and a small wire brush. If the damage is severe, replace the connector housing and/or individual pins. Ensure secure connections after reassembly.
4. Replace ECM/PCM: This should be a last resort. Only consider ECM/PCM replacement after absolutely ruling out all other possibilities related to the injector itself and its entire wiring circuit. ECM/PCM replacement typically requires programming or “flashing” to the vehicle’s specific VIN and options, which should be performed by a qualified technician using specialized equipment.
5. Post-Repair Verification: After any repair, clear the diagnostic trouble codes using a scan tool. Start the engine and allow it to reach operating temperature. Perform a road test under various driving conditions (idle, acceleration, steady cruise) to confirm the P1204 code does not return and that engine performance has been fully restored. Monitor live data for injector pulse width and fuel trim values to ensure proper operation.