* Wiring harness issues: A break, short, or poor connection in the wiring harness leading to injector #1 and #3 is a common cause. This could be a single point of failure affecting both, or separate issues in each wire. Check for continuity and proper grounding.
* ECU (Engine Control Unit) problem: The ECU is the "brain" of the engine. A faulty ECU could be failing to send the correct signal to activate those specific injectors. This could be due to a software glitch, internal hardware failure, or even a bad connection to the ECU itself.
* Injector driver circuit issue: The ECU doesn't directly control the injectors; it uses a driver circuit to amplify the signal. A problem in this circuit—specifically affecting the channels for injectors #1 and #3—would prevent them from firing.
* Grounding problems: A poor ground connection for the entire fuel injector circuit or specifically for injectors #1 and #3 could prevent them from receiving the necessary current to operate.
* Fuel injector control module (FICM) issue (diesel engines only): In some diesel engines, a separate FICM controls the injectors' high voltage requirements. A faulty FICM is a possibility if this applies to your engine.
* PCM (Powertrain Control Module) issue (some vehicles): Similar to the ECU, the PCM is a central computer that may control fuel injection. Problems here could lead to this specific failure.
* Fuse or relay failure: Although less likely to affect only two specific injectors, a blown fuse or a faulty relay in the fuel injector circuit could be the root cause.
* Sensor issues: Faulty sensors (like crankshaft position sensor, camshaft position sensor, MAF sensor) can cause the ECU to misinterpret engine conditions and alter fuel injection strategy, potentially leading to injectors being disabled.
Troubleshooting Steps:
1. Visual inspection: Check the wiring harness for any visible damage, corrosion, or loose connections, focusing on the wiring for injectors #1 and #3.
2. Continuity test: Use a multimeter to check the continuity of the wiring from the ECU/FICM to the injectors.
3. Voltage test: Check the voltage at the injectors while cranking the engine to see if the signal is reaching them.
4. Scan tool diagnostics: A scan tool can read diagnostic trouble codes (DTCs) from the ECU, which can pinpoint the exact problem. This is often the most efficient first step.
Without knowing the specific make, model, and year of the vehicle, it's difficult to give more precise advice. However, the above points are general considerations for why only specific injectors might not be firing despite being functional. Professional diagnostic help is often recommended when dealing with fuel injector problems.
