P1273

What Does Code P1273 Mean?

DTC P1273 signifies that the Engine Control Module (ECM), also commonly referred to as the Powertrain Control Module (PCM), has detected an open circuit condition within the electrical circuit of the fuel injector for cylinder #3. In modern engine management systems, the ECM actively monitors the electrical characteristics of critical components, including fuel injectors. Fuel injectors are typically supplied with a constant B+ (battery voltage, the “high side”), and the ECM controls the fuel delivery by rapidly grounding the “low side” of the injector circuit, completing the electrical path and energizing the injector’s solenoid coil. When the ECM commands the cylinder #3 injector to fire, it expects to see a specific range of current flow or a predictable voltage drop across the injector’s internal winding. An “open” circuit means there is a break or discontinuity in this electrical path—either in the injector itself, the wiring leading to or from it, or its connector—preventing current from flowing as expected. When the ECM detects an abnormally high resistance, or infinite resistance, indicating no current flow, it interprets this as an open circuit and sets the P1273 code. This condition directly impacts fuel delivery to cylinder #3, leading to a misfire and associated drivability issues.

Common Symptoms

• Check Engine Light (CEL) Illumination: The most common and immediate indicator.
• Engine Misfire: Noticeable rough running, especially at idle or under light load, due to lack of fuel combustion in cylinder #3.
• Reduced Engine Performance: Loss of power, hesitation, or stumbling during acceleration.
• Rough Idle: The engine may vibrate excessively or run erratically due to an imbalanced combustion process.
• Increased Fuel Consumption: While cylinder #3 isn’t receiving fuel, the ECM may attempt to compensate by altering fuel delivery to other cylinders or the overall engine efficiency is compromised.
• Foul Odor from Exhaust: Unburnt fuel passing through the exhaust system due to misfire, though less common with an open injector (no fuel injected). More often associated with a stuck-open injector or spark issue.

What Causes the Code P1273?

• Faulty Fuel Injector (Cylinder #3): The most prevalent cause. The internal winding of the injector’s solenoid coil can break or degrade, creating an open circuit within the injector itself.
• Open Circuit in Wiring Harness: A physical break, fraying, or severe corrosion within the wiring connecting the ECM to the cylinder #3 fuel injector. This can occur on either the power supply (high side) or the control/ground (low side) wire.
• Corroded, Loose, or Damaged Connector: The electrical connector attaching the wiring harness to the cylinder #3 fuel injector can become compromised due to corrosion, physical damage, or simply a loose connection, leading to an intermittent or constant open circuit.
• Faulty Engine Control Module (ECM/PCM): Although less common, an internal failure of the injector driver circuit within the ECM responsible for controlling cylinder #3’s injector can prevent the necessary power or ground signal from being supplied, effectively creating an open circuit condition from the module’s perspective.

How to Diagnose and Troubleshoot

Diagnosing P1273 requires systematic electrical testing using a Digital Multimeter (DMM) and an OBD-II scanner.

1. Initial Scan and Visual Inspection:
   • Connect an OBD-II scanner to confirm P1273 is present and check for any accompanying misfire codes (e.g., P0303).
   • Perform a thorough visual inspection of the cylinder #3 fuel injector and its wiring harness. Look for signs of physical damage, chafing, corrosion, or loose connections at the injector connector. Ensure the connector is fully seated and locked.
2. Fuel Injector Resistance Test:
   • Turn the ignition OFF.
   • Locate and disconnect the electrical connector from the cylinder #3 fuel injector.
   • Using a DMM set to ohms (Ω), measure the resistance across the two terminals of the fuel injector itself.
   • Compare the reading to manufacturer specifications (typically 10-16 ohms for most port fuel injectors). An infinite reading (OL or open loop on the DMM) or a reading significantly outside the specified range confirms an internal open circuit within the injector, indicating it is faulty.
3. Power Supply (High Side) Test:
   • With the cylinder #3 injector connector still disconnected, turn the ignition to the ON position (engine off).
   • Using the DMM set to DC volts, connect the negative lead to a known good ground (e.g., battery negative terminal).
   • Carefully probe one of the terminals in the injector harness connector. You should find battery voltage (approx. 12V) on one of the terminals. This is the constant power supply (high side) to the injector. If no voltage is present, check relevant fuses and wiring back to the power source.
4. Control/Ground (Low Side) Test:
   • This test confirms the ECM’s ability to provide a ground pulse.
   • Using a NOID light (a specialized test light designed for injector circuits) specific to your injector type, connect it across the two terminals of the injector harness connector.
   • Crank the engine. The NOID light should flash rapidly, indicating the ECM is attempting to pulse the injector by providing ground. If the light does not flash, there is an open circuit on the control side, or the ECM driver is faulty.
   • Alternative (less precise): Using a DMM set to AC volts (as the pulse is rapid and alternating), connect leads across the harness terminals. While cranking, you may observe a fluctuating voltage. More accurately, connect one DMM lead to battery positive and the other to the control side wire in the harness connector. During cranking, a good ECM signal will cause the DMM to show fluctuating voltage as the ECM grounds the circuit.
5. Wiring Harness Continuity Test (if injector is good and ECM signal is absent):
   • Turn ignition OFF and disconnect the battery.
   • Carefully disconnect the ECM/PCM connector. Consult a wiring diagram for the exact pinouts for cylinder #3 injector control and power.
   • Using the DMM in continuity mode, test for continuity between the cylinder #3 injector harness connector pins and their respective pins at the ECM connector.
   • Also, test each wire for shorts to ground and shorts to battery voltage by probing each wire and testing against chassis ground and battery positive (if applicable). An open wire or short will be identified here.

Recommended Repairs and Solutions

Once the diagnostic steps have identified the root cause, the following repairs are typically recommended:

1. Replace Faulty Fuel Injector (Cylinder #3): If the injector’s resistance test indicated an open circuit, replacement of the injector is necessary. Ensure proper O-ring lubrication and seating during installation to prevent fuel or vacuum leaks. It’s often advisable to replace injectors in sets (e.g., all injectors in one bank or all injectors) if the vehicle has high mileage, though for a specific open circuit, individual replacement is acceptable if the remaining injectors are functioning correctly.
2. Repair or Replace Wiring Harness: If a break, chafing, or corrosion is found in the wiring harness, repair the damaged section using appropriate soldering techniques, heat-shrink tubing, and secure routing. For extensive damage, replacing the entire harness section may be more practical.
3. Clean or Replace Connector: If corrosion or damage to the injector connector is identified, thoroughly clean the terminals using electrical contact cleaner. If the connector housing or terminals are severely damaged or loose, replace the connector with a new, high-quality component, ensuring secure crimps for new terminals.
4. Replace Engine Control Module (ECM/PCM): This is a last resort. If all other components (injector, wiring, connectors) test good and you’ve definitively diagnosed that the ECM is not providing the necessary control signal to the injector, then the ECM may require replacement and subsequent programming. This should only be performed after exhaustive testing by a qualified technician.

Mechanic’s Tips:

• Always clear the DTCs with an OBD-II scanner after any repair.
• Perform a thorough test drive under varying conditions to confirm the repair is successful and the P1273 code does not return.
• Consider performing a fuel pressure test to rule out other fuel system issues, especially if multiple misfire codes or fuel trim issues are present alongside P1273.
• When working with electrical connectors, especially those exposed to engine heat and elements, applying a small amount of dielectric grease can help prevent future corrosion.
• Ensure all wiring is properly secured and routed away from hot or moving engine components to prevent future damage.