Case Study: Lexus IS 250 MIL on, intermittently sets DTCs P0172 And P0175

Aug. 4, 2022

This vehicle was running well, but the malfunction Indicator Light (MIL) would occasionally come on. If the diagnostic trouble codes (DTCs) were cleared, after a few days, the MIL would come back on.

Richard Diegle

Related To: ALLDATA

Vehicle: 2012 Lexus IS 250, RWD, V6-2.5L (4GR-FSE), Automatic Transmission/Transaxle

Mileage: 139,007

Problem: This vehicle was running well, but the malfunction Indicator Light (MIL) would occasionally come on. If the diagnostic trouble codes (DTCs) were cleared, after a few days, the MIL would come back on.The problem started happening more often, so the owner brought it to the shop.

Case Details: The technician connected a scan tool and found these DTCs:

P0172 - System Too Rich (Bank 1)
P0175 - System Too Rich (Bank 2)

Both fuel trims were approximently 20 percent at idle but improved to nearly zero percent when cruising. The tech checked for an open EVAP purge valve, allowing fuel vapor into the manifold. It was sealed shut.

The technician made a quick call to ALLDATA Tech-Assist for some diagnostic advice. The Tech-Assist consultant recommended checking the fuel pressure, and the engine coolant temperature (ECT) sensor to be sure they were in normal range since both engine banks were experiencing the rich condition. He also advised testing the crankcase for a vacuum leak. The vacuum should be around 17-20".

NOTE: Not only will a failed PCV valve pull a large amount of oil and crankcase vapor into the motor, it can also cause a vacuum leak.

Confirmed Repair: The technician attached a vacuum gauge to the dipstick tube and found only 10" at idle. He replaced the PCV valve and cleared the DTC.Fixed!

Reprinted with permission from ALLDATA.

About the Author

Richard Diegle | Senior Automotive Editor for ALLDATA

Rich Diegle has been in the automotive industry since 1978. After graduating from a local community college, he began working at a Nissan dealership as a used car technician. Since then, he’s worked as a Nissan and ASE master automotive technician, shop owner, marketing copywriter, automotive editor, public relations manager, instructor, ALLDATA Tech-Assist team manager, and a senior automotive editor.