P1151 Code: Lack of HO2S21 Switch – Sensor Indicates Lean
🟠 Moderate severity · drivable
Drivable, but the engine runs lean on bank 2, which can cause hesitation and higher fuel use.
What P1151 means
The front oxygen sensor on bank 2 (HO2S21) keeps reporting a lean mixture and is not switching normally. The computer can't correct the mixture on that side of the engine, so bank 2 may run with too much air and too little fuel, hurting performance and fuel economy.
Symptoms
- Check engine light is on
- Rough idle or hesitation
- Loss of power on acceleration
- Poor fuel economy
- Failed emissions test
Common causes
- most common: Vacuum leak on the bank 2 side
- common: Faulty or aging oxygen sensor
- common: Exhaust leak before the sensor on bank 2
- less common: Low fuel pressure
How to fix it
| Fix | DIY? | Typical cost |
|---|---|---|
| Find and fix vacuum leaks | Yes | $0–100 |
| Replace the bank 2 front oxygen sensor | Yes | $60–150 |
| Repair the exhaust leak | Mechanic | $100–400 |
Frequently asked questions
Can I drive with a P1151 code?
Yes, but the lean condition can cause hesitation and poor economy. Get it fixed soon to avoid long-term damage.
What is bank 2 on my engine?
Bank 2 is the side of the engine that does not contain cylinder 1. The front sensor on that side is the one reporting the lean condition.
How much does it cost to fix P1151?
A vacuum leak fix can be free to about $100, and a new oxygen sensor is $60–$150 in parts.
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Updated 2026-08. This page is informational and not a substitute for professional diagnosis.