System Too Lean (Bank 1); Oftentimes, this trouble code is accompanied by an P0174
This is one of the most common trouble codes in V-6 and larger engines. P0171 is a "System Too Lean (Bank 1)," which is triggered by the first exhaust oxygen (02) sensor (aka upstream 02 sensor) on the "Bank 1" side of the engine (designated as whichever side has cylinder number one).
The Air-To-Fuel Mix
Nearly all Toyota gasoline engines strive for a 14.7:1 air to fuel mix, and they accomplish this goal by measuring the amount of oxygen in the exhaust stream using O2 sensors. The engine control module (ECM, sometimes referred to as a powertrain control module) uses readings from the O2 sensor to adjust the air/fuel mix for optimal efficiency. When the mixture has too much oxygen, the ECM adds more fuel to the engine.
If the oxygen readings don't change in a manner corresponding fuel mixture changes - i.e. the engine is running richer, but the oxygen sensor(s) aren't measuring any change in oxygen levels - the ECM will register a P0171 OBD II code. Quite often P0171 is triggered in tandem with P0174, which comes on when the Bank 2 sensor detects too much oxygen in the exhaust.
Troubleshooting P0171 Code
Oftentimes, a P0171 means that your Toyota is having trouble with a dirty or faulty mass airflow (MAF) sensor, or a vacuum leak downstream of the MAF sensor (perhaps in the PCV line or in the valve itself). Of course, this code can also signify a bad O2 sensor in bank 1.
Additionally, it's rare (but still possible) that a P0171 code can be tripped by a plugged fuel injector, by a dirty fuel filter causing low fuel pressure, or even an exhaust leak before the first O2 sensor. Isn't engine trouble code diagnosis fun?
- Most of the time, the best way to respond to a P017 is to try cleaning the MAF sensor and the lines running to and from it. This will often resolve this issue.
- When you clean the MAF sensor, be sure to inspect all of the lines including the vacuum and PCV hoses. Look for kinks, cracks, etc., and replace the lines as needed. Additionally, be sure to check all of the connections and hoses on the air intake and check for intake manifold leaks.
- Finally, it's never a bad idea to change your air and fuel filter, especially if they're just about due for replacement anyway.
If cleaning the MAF sensor and inspecting the lines doesn't solve the problem, it may be time to replace the O2 sensor on bank 1. Consult your Toyota's manual for its exact location, which is specific to the model. If that doesn't do it, it's probably time to enlist the help of a professional.
For a comprehensive list of trouble codes, check out this article on the Toyota Parts website.