P1157
Air/Fuel Ratio (A/F) Sensor 1 Range/Performance Problem
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The P1157 code is a Honda-specific diagnostic trouble code indicating that the Air/Fuel Ratio (A/F) Sensor 1 — the upstream oxygen sensor before the catalytic converter — is reading outside its expected performance range. This sensor is critical for the ECM's ability to manage the air-fuel mixture. When it sends inaccurate data, the engine may run too rich or too lean, leading to poor fuel economy, rough running, and increased emissions.
The most common cause on Honda vehicles is a failing A/F sensor itself. These sensors degrade over time from heat exposure and contamination. However, it's important to also check the wiring and connectors leading to the sensor, as corrosion — especially in areas with road salt — is a frequently reported issue on Honda forums. Exhaust leaks near the sensor can also produce misleading readings.
If you're comfortable working under the vehicle, replacing the A/F sensor is a straightforward job requiring an oxygen sensor socket. Before replacing the sensor, inspect the wiring harness for obvious damage and check for exhaust leaks. If the code returns after sensor replacement, a deeper investigation into vacuum leaks or fuel system pressure may be needed.
Severity
Symptoms
- •Check Engine light illuminated
- •Reduced fuel economy
- •Rough or uneven idle
- •Hesitation or stumbling during acceleration
- •Occasional black smoke from exhaust
- •Engine running rich or lean intermittently
Likely Causes
The primary A/F sensor (upstream oxygen sensor) may have degraded over time, producing inaccurate voltage readings that fall outside the expected range.
Wires leading to the A/F sensor can become corroded, chafed, or broken due to heat and road exposure, causing erratic signal output.
An exhaust leak upstream of the sensor introduces outside air, skewing the sensor's oxygen readings and causing false lean indications.
Unmetered air entering through cracked vacuum hoses or a weak fuel pump can cause a genuinely lean condition that the sensor reports accurately.
Estimated Cost
Professional Repair
Includes parts + labor
Common Fixes
- Replace the upstream air/fuel ratio sensor (sensor 1)
- Repair corroded or damaged wiring and connectors at the A/F sensor
- Fix exhaust leaks near the exhaust manifold or sensor location
- Inspect and repair vacuum leaks