Lack of HO2S21 Switch - Sensor Indicates Rich

What it means

This is a Ford, Lincoln and Mercury code; the same number can mean something different on other makes. The upstream oxygen sensor HO2S21 on bank 2 of V6 and V8 engines stayed rich and did not switch lean. If P1132 is also stored the cause is usually shared by both banks; if it appears alone, the cause is most likely on the bank 2 side.

Models covered: Ford (and Lincoln/Mercury) models with V6/V8 engines, mainly around 1996–2004.

Related codes: P1151P1132P0175P0152

Symptoms4

• Check engine light on.
• Higher fuel use.
• Fuel smell from the exhaust at times.
• P0175 may be stored with it.

Possible causes5

• Leaking injector on bank 2.
• High fuel pressure when both banks are affected.
• EVAP purge valve stuck open when both banks are affected.
• Wrong reading from the MAF sensor or the coolant temperature sensor.
• Contaminated or failed HO2S21 sensor.

How to diagnose4

1. Compare fuel trims on both banks to see whether the problem is one-sided or shared.
2. Check bank 2 injectors by listening, balance testing or leak testing.
3. Measure fuel pressure and check the EVAP purge valve when both banks are affected.
4. Test HO2S21 response by briefly adding air; if the reading does not drop, it is contaminated.

How to fix5

1. Repair or replace the leaking injector.
2. Repair the fuel pressure regulator or purge valve when both banks are affected.
3. Clean the MAF or replace the temperature sensor based on the test.
4. Replace the HO2S21 sensor if proven faulty.
5. Clear the code and confirm fuel trims return to normal.

Repair cost

In Saudi riyals; the highlighted part is the expected range for this fault.

The fix starts with a cheap valve or sensor, and rises for injectors or an oxygen sensor on the harder-to-reach side.

Prices are rough estimates for common cars at independent workshops and vary with the car and the source of the part.
Prices last updated: September 2026

Frequently asked questions