Post Catalyst Fuel Trim System Too Lean (Bank 1)

What it means

The engine computer uses the oxygen sensor after the catalytic converter to fine-tune the mixture. It has found that this sensor keeps reading lean and the correction has reached its limit. The most common cause is an exhaust leak near the sensor or a fault in one of the oxygen sensors.

Related codes: P2098P2097P0171P0137P0420

Symptoms4

• Check engine light on, often with no other symptoms.
• Sometimes a slight increase in fuel consumption.
• Ticking or puffing sound from the exhaust if there is a leak.
• Failed emissions test.

Possible causes5

• Exhaust leak ahead of the downstream oxygen sensor letting outside air in.
• Faulty or slow downstream oxygen sensor.
• Aged upstream oxygen sensor with a reading biased toward rich.
• A real lean mixture from a vacuum leak or low fuel delivery.
• Wiring or ground fault in the downstream sensor circuit.

How to diagnose5

1. Read all stored codes and fix mixture or oxygen sensor codes such as P0171 or P0137 first.
2. Inspect the exhaust from the manifold to the downstream sensor for leaks or damaged gaskets.
3. Watch the downstream sensor reading and fuel trims on a scan tool while driving.
4. Compare how the upstream and downstream sensors respond to a sudden rich command to find a slow sensor.
5. Inspect the downstream sensor wiring and connector.

How to fix5

1. Repair the exhaust leak or replace the damaged gasket.
2. Repair the sensor wiring or connector if a fault is found.
3. Replace the downstream oxygen sensor if it is slow or stuck.
4. Replace the upstream oxygen sensor if its reading is proven biased.
5. Clear the codes and confirm the trims return to normal.

The cost is low for a simple exhaust leak repair, and rises when a new oxygen sensor or a fuel system repair for a real lean condition is needed.

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