What Does P1109 Mean?
P1109 indicates the Bank 2 Sensor 2 oxygen sensor circuit is reporting voltage that is too low, or there is an air leak affecting the downstream O2 sensor reading. The post-catalyst O2 sensor on Bank 2 is showing a persistent lean signal, which could mean the sensor itself is biased lean, there is an exhaust leak near the sensor, or the catalytic converter on Bank 2 is not functioning properly.
Common Causes
30%
Exhaust leak near the Bank 2 Sensor 2 location drawing in ambient air
28%
Failing O2 sensor biased lean or slow to respond
20%
Deteriorated catalytic converter on Bank 2 causing abnormal O2 readings
12%
Wiring issue causing voltage drop in the O2 sensor signal circuit
10%
Contaminated O2 sensor from coolant or oil intrusion
Diagnostic Steps
1
Step 1: Monitor Bank 2 Sensor 2 voltage at idle. A healthy post-cat sensor should show a relatively steady voltage (0.5–0.8V). A consistently low voltage (below 0.3V) suggests lean bias or sensor failure.
2
Step 2: Inspect the exhaust system near the Bank 2 Sensor 2 bung and the catalytic converter outlet for leaks. Use a smoke machine or propane enrichment test to detect small leaks.
3
Step 3: Compare Bank 1 Sensor 2 and Bank 2 Sensor 2 readings. If Bank 2 is significantly lower, the problem is isolated to that bank rather than a system-wide lean condition.
4
Step 4: Check the O2 sensor connector and wiring for damage, corrosion, or high resistance. Verify the sensor ground path is clean and low resistance.
5
Step 5: If no exhaust leak is found and wiring checks out, replace the Bank 2 Sensor 2 O2 sensor. Clear codes and run a drive cycle to verify proper catalyst monitoring.
Estimated Repair Cost
$75 - $400
Parts + labor, varies by vehicle and location
P1109 indicates the Bank 2 Sensor 2 oxygen sensor circuit is reporting voltage that is too low, or there is an air leak affecting the downstream O2 sensor reading. The post-catalyst O2 sensor on Bank 2 is showing a persistent lean signal, which could mean the sensor itself is biased lean, there is a...
The most common cause of P1109 (O2 Sensor Circ.,Bank2-Sensor2 Voltage too Low/Air Leak) is: Exhaust leak near the Bank 2 Sensor 2 location drawing in ambient air
Typical repair costs for P1109 range from $75 to $400, depending on the vehicle, location, and whether you do it yourself or go to a shop.
This fault affects emissions monitoring and catalyst efficiency detection. An exhaust leak can introduce carbon monoxide into the cabin if located near firewall penetrations, which is a potential health hazard.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to P1109 to identify the root cause.
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Quick Info
Category
Powertrain
System
Emissions / Exhaust System
Difficulty
Type
Manufacturer
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