I own a 2001 Hyundai Sonata. Check engine light is on, and I'm getting an error code P0171, Bank 1 System too lean.

I figured I'd start eliminating problems, so when I ran an ODB check, what I got was a lot of monitors were in Ready mode, except the following:

  1. EVAP
  2. O2S
  3. CAT

I've driven around countless miles, but somehow I can't get these monitors in ready. The first thing I've started with was checking the O2 sensor. I can't figure out which O2 sensor I'm checking, and I don't know where Bank 1 is. I'm not sure if I pulled out the right sensor, but I think the sensor before the converter would be bank 1?

Images are provided: (This is a sensor that I haven't yet pulled out just yet)

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This is the sensor I think that won't go in ready mode, but I'm not sure:

enter image description here

The above sensor was a sensor right below the sensor in the first picture, going into a pipe, I'm assuming the exhaust to the converter.

Is it possible you guys happen to know what 02s I need to check? Again, it shows the O2S not in Ready mode, and I've driven for a while and it still isn't on.

The error code is P0171 Bank 1 System Too Lean

  • Bank 1 usually refers to one side of an engine or the other on a V6, or if a 4-cyl, there is only one bank. Usually a code will read bank 1 sensor 1 (B1S1) for the before cat and bank 1 sensor 2 (B1S2) for after cat. Also, in order to get it into ready mode, you will most likely have to go through several heat/cool cycles. Commented Aug 5, 2014 at 1:56
  • @Paulster2 Alright, but when i run the scan using an ODB, it shows that a o2s isn't ready. Would you happen to know what sensor this is referring to? Commented Aug 5, 2014 at 1:59
  • It is just telling you both (assuming 4-cyl) O2 sensors are not ready. Until you go through several cycles, you won't know if one or the other is bad. Commented Aug 5, 2014 at 2:03
  • @paulster2 I've been through so many, and it still isn't showing in ready mode. However my Ho2s is in ready mode. the one I pulled out, is that an o2s or an Ho2s? if you can tell? Commented Aug 5, 2014 at 2:14
  • In a straight-N engine, Bank 1 is the only bank you get in OBD or OBDII - nothing is Bank 2. The O2 sensor upstream of the cat is O2 Sensor 1, the downstream one is Sensor 2. Sensor 2 in your case may never reach a "ready" condition, and the cat may never reach a "ready" condition, because your system is running too lean to feed the catalyst. Let's see about getting your lean condition fixed. Fuel pump, pressure regulator, injectors. Commented Aug 5, 2014 at 3:22

1 Answer 1


Bank 1 on any engine is the bank that carries No 1 cylinder. Sensor1 is the first sensor from where the exhaust manifold bolts onto the engine/head. Some scans will show an 'N/A' on a four cylinder engine for Bank2. This means that the ECU is programmed for a four cylinder, but can be programmed for a 6 or 8 cylinder vee using differant software. Your vehicles ECU software monitors must all run to set all components to Ready, this is called a drive cycle. Your EVAP system is tested by a leakage test with a scanner, which will also check the systems purge valve and pressure sensor. Any system leaks can be found with a smoke test. The EVAP system needs the O2 sensor to be working correctly on a drive cycle to set the EVAP to Ready. The O2 sensor can be tested by a propane test. The Cat uses a second dedicated sensor after the Cat to monitor the Cat's performance, to pass the Cat to Ready. Correct testing of components in essential, anything else is a guess.

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