06/05/2026
Codes P0171 (Lean Bank 1) & P0175 (Lean Bank 2) are probably the two most common emissions codes we deal with on a daily basis.
The fundamental failure is that the Oxygen Sensor measured a high oxygen content in the exhaust. The sensor output voltage was low. But WHY??
Is the sensor defective, or as in most cases, is there truly a high oxygen content in the exhaust?? Is there an exhaust leak scavenging outside air (oxygen) into the exhaust? Is there a vacuum leak?
I cannot tell you how many times I have been told, "My car's computer said the Oxygen Sensor is bad". No, it didn't. It never tells us what part to replace. The code tells us what test failed. P=Powertrain, 0=Universal, 1=Fuel System, 71=Failed Test.
Air entering the engine is measured by weight. For every 14.6 grams of air entering a cylinder, 1 gram of gasoline is injected. With a vacuum leak, the most common failure is air leaks around the inlet air sensor (False Air) and the actual weight is under-measure.
When that happens, the computer underdelivers fuel, causing high oxygen content in the exhaust (Lean Condition).
Vacuum leaks can sometimes be very difficult to find. At Complete Car Care, we use compressed air & smoke to locate those hard-to-find vacuum leaks.
With this big moving van, the leak was under the Intake Manifold and hard to detect. I replaced one defective vacuum hose and the engine stalling complaint was gone. No Oxygen Sensor is needed. Good technology and a complete diagnostic are essential to quality repairs. Hope this helps.
Warren Parr, Owner. Complete Car Care. Serving our community for 40 years
