A P0171 check engine light (CEL) on a Scion tC indicates that the ECU has detected a “System Too Lean” condition on Bank 1 — meaning the air-fuel mixture contains too much air and not enough fuel. This imbalance can be caused by several issues in the fuel delivery, air intake, or sensor systems.
Possible causes of a P0171 check engine light
Vacuum leak, which allows unmetered air to enter the intake manifold. Cracked or disconnected vacuum hoses, a faulty intake manifold gasket, or a leaking PCV hose can all create this issue.
Dirty or faulty MAF sensor is another frequent culprit. Over time, oil, dirt, or debris can accumulate on the sensor’s element, causing it to misread the amount of incoming air and send inaccurate data to the ECM. Cleaning or replacing the sensor often resolves this.
Fuel delivery issues can also cause a lean condition. A clogged fuel filter, weak fuel pump, or faulty fuel pressure regulator may restrict fuel flow, reducing pressure at the injectors. Similarly, dirty or partially clogged fuel injectors can fail to deliver enough fuel to the combustion chambers.
Exhaust leaks before the oxygen sensor can cause the sensor to detect extra oxygen, falsely signaling a lean condition.
Diagnosing a P0171 code involves checking for vacuum leaks, cleaning or testing the MAF sensor, inspecting fuel pressure, and verifying sensor readings.
Tools Used
I started w/ the cheapest attempt to fix first
I did not opt to clean the MAF sensor. I decided to replace the MAF with a cheap non OEM brand with the best reviews on Amazon. Low risk here because I can always return it. I’m OK with this being that an Original OEM Toyota MAF sens4nQwThkor costs almost $150 from the dealer.
LEFT: OEM Mass Air Flow Sensor 22204-22010
RIGHT: Aftermarket NON OEM Mass Air Flow Sensor 22204-22010
Testing for leaks with a smoke machine
cheap $42 evap leak tester smoke compressor from Amazon.
Kit comes complete with 3 packs of smoke, hang chain, power cables, pippet and hose
Seal off the insert with a rubber glove and tape
Found the leak, busted PCV to Intake manifold hose. This leak was entirely my fault.
The aftermarket PCV to Intake catch can I installed requires me to change out the container out around every 500 miles (yes that often).
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The quality of the replacement hose (Amazon, not OEM) was poor so it got brittle and cracked.
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The fit was too tight causing the hose to stretch at the ends.
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The constant removal of the catch can to dump blow off oil lead to premature wear
I decided to replace the PCV to intake hose with Fuel rated hose. I got 2′ of hose from my local Oreilly’s for about $9.50.
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Thicker wall
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Correct ID (7/16″) to compensate for the larger nipple on the intake side
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Added more slack to allow for more play/wiggle room when removing and installing the catch can reservoir
If available use a hose cutter to get a nice flush cut on the hose end. You can see here it makes a straight cut
Final thoughts
After clearing the P0171 CEL code, I need to complete a drive cycle so that the incomplete monitors clear. If you decide to install a catch can between your PCV valve and intake manifold, use heavy duty hose to prevent future leaks in this area. It’s very common, but my particular issue was self induced being that I added a high traffic part there – the need to constantly dump the catch can reservoir led to pre mature wear on the hose and eventually cracked it.
