Rick's Free Auto Repair Advice

P0171 trouble code

P0171 trouble code System too lean (Bank 1)

If you have a P0171 trouble code System too lean (Bank 1) trouble code, read this to understand what it means

Lean codes are incredibly difficult to diagnose. And it’s one trouble code where you don’t want to start throwing parts at the problem. The guys at the parts store will tell you to replace the Oxygen sensor. Nice guess, but probably wrong. They’re assuming that the fuel mixture is correct but the sensor is reporting it wrong. Oxygen sensors DO wear out. But what if the mixture really IS lean? Can it be lean on only one bank (of a V-6 or V-8) engine? You bet. This kind of code screams out for a little diagnostic work. First, check all the vacuum hoses for cracks and correct placement. Next, spray carburetor cleaner around the intake manifold gasket and listen for an increase in engine RPM. Any increase after spraying indicates a vacuum leak.

The Bank 1 indication tells you that the problem is located on the cylinder bank that contains the #1 plug. That varies by make and engine style, so make sure you check the shop manual.

Most common causes of P0171 trouble code


NOTE: This information is also applicable to code P0174, system too lean bank 2, as this code is the same except of the opposite bank of cylinders.

Most common

• Dirty Mass Airflow Sensor
• Vacuum Leak
• Bad Oxygen Sensor(s)
• Low Fuel Pressure
• Running Out of Gas

Less Common Causes

• Clogged Fuel Injectors
• Leaking Exhaust System
• Evaporative Emissions System Malfunction
• EGR System Malfuncion

Fix P0171 trouble code

Check fuel level. Really low fuel can cause a P0171 trouble code simply because the engine isn’t getting enough gas.

Next, clean the MAF sensor. If it’s sending the wrong reading, the PCM may not be providing enough gas for the actual amount air entering the engine.

Check for vacuum leaks. Check for tears in the air duct after the

MAF sensor and before the throttle body. These tears are fairly common on some cars. If the air duct checks out, check for engine vacuum leaks.

Check Oxygen Sensors

This is somewhat difficult to do for DIYers because the most accurate method is with a digital scope. However, if you have a scan tool with live data, you can check the oxygen sensor readings.

Then check fuel pressure. Low fuel pressure can cause a lean condition simply because the cylinders aren’t getting enough gas.

© 2012 Rick Muscoplat

Posted on by Rick Muscoplat

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