Fault code P0171 – a step-by-step guide to diagnosing and fixing it

The DTC indicates that the oxygen sensor on the first cylinder bank has detected lean engine conditions. This means that there is too much air or not enough fuel in the system. The ECU adjusts the air-fuel mixture based on the sensor readings to achieve the optimum ratio (14.7). If it is unable to maintain the correct ratio by injecting more fuel, the computer will register this code. P0171 is often accompanied by P0174 (System Too Lean Bank 2).

• A contaminated or faulty mass air flow sensor
• A vacuum leak
• Sensor malfunctions or wear
• A stuck PCV valve
• A clogged or broken car fuel injector
• Low fuel pressure
• Exhaust leaks

• A loss of engine power
• Starting problems
• An increase in emissions
• Lower fuel efficiency
• Engine overheating
• Rough idling
• Stalling or difficulty accelerating
• A popping sound in the car exhaust pipe
• The “Check Engine” light is illuminated on the dashboard

Before tackling repairs, the very first thing you will need to do is a diagnostic scan. You can scan the computer using an OBD2 scanner and check the related trouble codes. This will not only help you to identify the root cause of the fault, but could also reveal other hidden problems.

Depending on what you find, the following components may need to be inspected:

• The vacuum hoses. These include the vacuum lines, air intake pipe, fuel pressure regulator hose, and so on. You should look for any signs of wear, damage or leaks. Leaks can usually be detected using a smoke generator, although not everyone has access to this equipment.
• The exhaust pipe. The pipe and exhaust manifold gasket should be inspected for leaks as this will directly affect the oxygen readings.
• Sensor wires and connectors. First, a visual inspection can be carried out to determine whether the wires have burnt out, parts have corroded, connections are loose or the pins are bent. You may also need to check the device’s electrical resistance and voltage using a multimeter. In some cases, a bad connection can be repaired by simply removing traces of corrosion and contaminants from the connectors.
• The MAF sensor. This component sometimes needs to be cleaned as dirt, oil, dust and other substances can affect its resistivity. Specialised cleaning agents, such as those made with methanol, can remove contaminants including carbon deposits without damaging the parts. You can find detailed guides on how to test MAF devices online.
• The fuel injection system. Components such as the pump, pressure regulator, and injector nozzles may need to be checked with a multimeter or manometer. A clogged fuel filter may also be the cause of low fuel pressure.

Once you have obtained a proper diagnosis, you can repair the units by cleaning, tightening, and replacing the faulty components. Don’t forget to check the gaskets too as they may need to be replaced to fix or prevent leaks.