Fixing a Check Engine Light (P0171)




Introduction: Fixing a Check Engine Light (P0171)

Avid DIYer, tinkerer

Your check engine light just came on! And your worried sick. However, this is no reason to panic. The following instructions will show you how to troubleshoot a common check engine light issue found mainly in Toyota and Honda cars.

Step 1: Purchase a OBD 2 Bluetooth Scanner

The Bluetooth scanner runs around 15 dollars on Ebay (A typical autoshop will charge you more then 50 dollars just to perform a scan). After purchasing the scanner download the Torque (lite) app.

Step 2: Insert the Scanner Into the OBD 2 Port of Your Car.

The OBD 2 port is generally located underneath your steering wheel and above the gas pedal. After inserting the scanner turn on your Bluetooth and open the Torque app.

Step 3: Run a Diagnostic Scan

Go to settings then click "fault codes" and a diagnostic scan will be preformed.

Step 4: P0171

After the scan the fault codes will be available. The P0171 is a common fault code among Toyota (In this case a 2001 Toyota Corolla). The code means that you are running lean (i.e. too much oxygen in your exhaust). If the scanner reads another fault code you can proceed to troubleshoot your issue using other resources online if your are comfortable doing so (at the very least you will know what is wrong with you car before taking it to a shop).

Step 5: Removing the Mass Air Flow Sensor

The MAF sensor measures the amount of 02 entering the engine therefore telling the cars computer how much gasoline to inject to ensure the proper ratio. If this sensor becomes dirty the air/02 to gasoline ratio can be scewed (i.e. too lean). The sensor is located just after the air filter and before the throttle body. The MAF can be removed by unscrewing the two screws and disconnected the electronic connection.

Step 6: The MAF Sensor

This is what the sensor looks like!

Step 7: Clean the Sensor

Clean the sensor thoroughly (8-10 sprays) using CRC Mass Air Flow Sensor cleaner. After the sensor is clean and dry reinstall it and connect the electronic connection.

Step 8: Clear the Code

Cleaning the MAF should resolve the issue however the check engine light will need to be cleared. To do this open the Torque app: setting >fault codes> clear logged fault(s)>yes.

Step 9: No More Check Engine Light.

The engine code has been cleared and once the cars computer resets to current conditions (typically after 100+ miles) you will be able to determine if your troubleshooting efforts paid off. Hopefully this resolves your issue and you were able to cheaply (just the cost of the scanner and spray) fix your car!

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    5 years ago

    You can buy a bluetooth OBD2 interface for around $4,- on Ebay. search for ELM 327. Works perfectly!


    5 years ago

    I have a 2008 Nissan xterra. This summer the CEL came on and I did not have an OBD 2 scan tool. Purchased a simple one for about $20 off of Ebay. It gave me a code that indicated an EVAP valve failure. I cleared the code thru the use of the code reader and of course the code came back again. Research indicated that the solenoid located on a charcoal canister within the rear quarter panel near the gas fillerd contained the valve which often fails on the Nissan xterra due to dust and grit getting into the breather line which Nissan installed in the frame of the underside of the vehicle. Removal and replacement of the valve with a new solenoid was a real challenge that is beyond words. Cleaning of the solenoid did not work initially which resulted in a repeat removal of evap lines and components and the replacement with a new solenoid. With all of the rust particles from the underside of the vehicle, one managed to drop in the electrical connector of the new solenoid which resulted in another CEL (check engine light) code.....evap solenoid shorted. This required removal again and investigation of the wiring to the solenoid. By accident I discovered the rust particle in the housing of the solenoid which shorted the terminals when the unit was to operate. Cleaning and replacement of the solenoid solved that CEL problem. The evap solenoid does nothing to effect the drivability of the car. It only tests the operation of the Evap system to trap and recycle gasoline fumes in feed it back to the running engine. The new code P0456 small evap leak indicated a pin hole sized leak either at the solenoid valve or the hard plastic hose line connectors.
    Removal and inspection of lines went on for some time with the code coming on at different times. After awhile I started to investigate the gas cap issue. Tightening didn't seem to matter. I could go different amount of miles or days and the infamous CEL would appear. One day I decided to try a coating of wheel bearing grease on the rubber seal of the gas cap and a thin coating on the mating surface of the gas fill tube. Voila.....the CEL was solved. I am sure all of this could have been solved at a dealership for about $1K and multiple trips to the dealership. In the midst of all this, my vehicle needed a state inspection and the OBD 2 code reader allowed me to clear the code from the computer (which at the time was probably the faulty gas cap seal) and allow the car to pass inspection. The OBD 2 scan tool is great, but don't expect it to tell you exactly what the problem is and how to fix it. Be prepared to get educated, dirty and frustrated if you want to fix your vehicle. Most of all, luck and magic inspiration often helps in the diagnosis and repair.


    5 years ago

    Have been considering getting an OBD2. This has just about sold me. Thanks OP ?


    5 years ago

    Oftentimes it can be a simple thing that triggers the dreaded check light- I had an Elantra that had a loose gas cap which triggered it, simply tightening the cap until sealed cured it in about 100 miles, then as you say- lights out! ☺