P1138: O2 Heater Circuit Slow Response Bank 1 Sensor 1
The P1138 code indicates that the powertrain control module (PCM) has detected insufficient switching activity from the heated oxygen sensor (HO2S) located before the catalytic converter on bank 1. During closed-loop operation, the sensor should rapidly alternate between rich and lean states, but the PCM has determined that the switching rate is too slow or infrequent. This prevents the PCM from properly adjusting the air-fuel mixture, leading to increased emissions and reduced engine performance.
01. Most Common Causes
- Faulty or degraded oxygen sensor (HO2S) due to contamination from silicone, lead, or carbon buildup
- Open or short circuit in the oxygen sensor heater circuit
- Damaged, shorted, or high-resistance wiring in the O2 sensor circuit
- Corroded, damaged, or bent pins in the O2 sensor electrical connector
- Blown fuse for the O2 sensor heater circuit
- Exhaust leaks upstream of the O2 sensor allowing false air
- Incorrect fuel pressure or dirty fuel injectors causing a constant lean or rich condition
- Faulty PCM (rare)
02. Symptoms You Will Notice
- Check Engine Light illuminated
- Poor fuel economy
- Rough idle or hesitation during acceleration
- Failed emissions test
03. Step-by-Step Diagnosis
Scan for codes and record freeze frame data
Inspect the O2 sensor connector and wiring for damage, corrosion, or loose connections
Check the O2 sensor heater circuit fuse and verify power and ground at the sensor connector
Test the O2 sensor heater resistance with a multimeter (typically 4-10 ohms)
Monitor O2 sensor voltage with a scan tool; should switch between 0.1V and 0.9V at 2500 RPM
Check for exhaust leaks upstream of the sensor
If all else passes, replace the O2 sensor and clear codes
1. Frequently Asked Questions
Is it safe to drive?▼
Yes, you can usually drive short distances, but the vehicle may run poorly, have reduced fuel economy, and could fail an emissions test. Prolonged driving may damage the catalytic converter.
How much does it cost to fix?▼
Typical repair costs range from $150 to $400, including parts (O2 sensor $50-$150) and labor (1-2 hours at $100-$150 per hour).