This code indicates that the upstream heated oxygen sensor on bank 2 (HO2S21) is reporting a consistently lean air-fuel mixture and has stopped switching between rich and lean states as expected. The condition may be caused by an actual lean running condition, unmetered air entering the exhaust stream, or a sensor that has failed with a lean bias.
Unmetered air entering through cracked vacuum hoses, leaking intake manifold gaskets, or faulty PCV components causes the engine to actually run lean on bank 2.
Cracks or gaps in the exhaust manifold or piping before the sensor allow outside air to enter, introducing oxygen that the sensor reads as a lean condition.
The sensor element may be contaminated or worn out, causing slow response times or a permanent lean bias that prevents proper switching.
Low fuel pressure, a restricted fuel filter, or partially clogged injectors specific to bank 2 can create a genuine lean combustion condition.
Corroded, chafed, or poorly connected wiring in the sensor circuit can cause voltage readings that mimic a lean condition.
A rare internal PCM fault affecting the sensor input circuit; this should only be considered after all mechanical, fuel system, and wiring causes have been eliminated.
No. While a failed sensor can set this code, vacuum leaks and exhaust leaks that introduce unmetered air are statistically more common causes than the sensor itself.
Short trips are possible, but prolonged operation may overheat and damage the catalytic converter due to excess oxygen; repair should be prioritized.
Almost never. The PCM is rarely the root cause of this specific code; it should only be considered after verifying no intake or exhaust leaks, confirming sensor and wiring integrity, and ruling out fuel delivery issues.
An exhaust leak upstream of the sensor draws in outside air, which contains oxygen; the sensor detects this excess oxygen and reports a lean condition regardless of actual combustion mixture.