O2 Sensor Signal Stuck Rich (Bank 1 Sensor 1)

What it means

The engine computer has found that the upstream oxygen sensor on bank 1 keeps reporting a rich mixture and does not change even when the computer cuts fuel. The mixture may really be rich from excess fuel, or the sensor may be contaminated or its wiring faulty.

Related codes: P2195P2198P0132P0172P0133

Symptoms4

• Check engine light on.
• Loss of performance and occasional stumbling because the computer cuts fuel.
• Fuel smell or black smoke from the exhaust if the mixture is truly rich.
• Failed emissions test.

Possible causes5

• Upstream oxygen sensor contaminated by oil, coolant or excess fuel.
• A real rich mixture from a leaking injector or high fuel pressure.
• EVAP purge valve stuck open.
• Sensor signal wire shorted to a voltage source.
• Sensor failure from age.

How to diagnose5

1. Read all stored codes; rich codes such as P0172 point to real excess fuel.
2. Watch the sensor signal on a scan tool, then briefly create a vacuum leak by pulling a vacuum hose; a good sensor swings lean immediately.
3. Watch the fuel trims; large negative values show the computer is trying to cut fuel.
4. Measure fuel pressure and test the purge valve.
5. Remove the sensor and inspect its tip for white or black deposits, and check its wiring.

How to fix5

1. Replace the EVAP purge valve if it is stuck open.
2. Repair shorted sensor wiring.
3. Repair the leaking injector or pressure regulator if the mixture is truly rich.
4. Replace the upstream oxygen sensor after fixing the source of contamination.
5. Clear the codes and confirm the sensor responds normally.

The cost is low for a purge valve or wiring repair, and rises when the sensor or injectors are replaced or when oil or coolant entry must be fixed.

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

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