Fault code P0174 – a step-by-step guide to diagnosing and fixing it

P0174 is triggered when the upstream oxygen sensor on cylinder bank 2 indicates that the conditions are too lean in one of the cylinders. This means that the cylinder has too much air or not enough fuel according to the optimal air-fuel ratio. When the sensor readings show there is too much oxygen, the powertrain control module compensates by adding more fuel. However, once it reaches the limit for added fuel, the system registers the code. This will affect the vehicle’s fuel economy and engine performance.

• It is often caused by low fuel pressure, occurring due to a faulty fuel pump or broken fuel pressure regulator in the car.
• There may also be a blockage in the fuel delivery system, caused by a dirty filter, clogged injectors or blocked fuel lines.
• Sometimes the culprit is a vacuum leak in the intake or exhaust system, releasing more gases into the cylinders. For example, it could be due to a leaking damaged manifold gasket or cracked air hoses.
• If the car's mass air flow (MAF) or oxygen sensor malfunctions, the device can transmit false signals to the control unit, triggering the DTC.
• If the exhaust gas recirculation (EGR) valve or positive crankcase ventilation valve is stuck in the open position an excess of gas will reenter the engine.

• Increased fuel consumption
• Activation of the check engine light
• A loss of engine power
• A misfiring engine
• Poor acceleration
• A rough idle
• Stalling

• Check the car’s fuel pressure by installing a pressure gauge and running the pump. If the reading is below the standard pressure values, the pump and regulator device should be inspected and repaired. To restore the pump, you can disassemble the component before calibrating it and using an ultrasonic cleaning system. The seals and other parts of the components may need to be replaced. After repairing it, the device should be tested at a test bench. If the regulator has gone bad, you will need to install a new one.
• Inspect the intake and exhaust systems for leaks. Replace any damaged hoses, lines and seals.
• The valves may be jammed due to a build-up of carbon deposits or other contaminants. In this case, you may be able to fix them using specialised cleaning sprays.
• You may need to flush the diesel or petrol fuel injectors and lines to ensure that there is a reliable supply of fuel to the combustion chambers. This can be done with special cleaning agents or an ultrasonic cleaner.
• Test whether the sensors are working properly with a multimeter. If a problem is detected, the solution could be as simple as cleaning the components or replacing the wires.