P1159
Air/Fuel Ratio (A/F) Sensor 1 AFS+ Terminal Low Voltage
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The P1159 code is a Honda-specific code indicating that the Air/Fuel Ratio Sensor 1's AFS+ terminal is reading low voltage. This terminal carries the primary signal from the upstream A/F sensor to the ECM. When voltage drops below the expected threshold, the ECM cannot properly determine the exhaust gas oxygen content, which compromises its ability to manage the fuel mixture effectively.
This code is closely related to P1157 and may appear alongside it. The root cause is often a failing sensor, but electrical issues are equally common. Corroded connectors and damaged wiring are frequent culprits, especially on higher-mileage Honda vehicles where the sensor harness has been exposed to years of heat cycling and road debris.
Diagnosis should begin with a visual inspection of the A/F sensor connector and wiring harness. Use a multimeter to check the voltage at the AFS+ terminal with the engine running — it should fluctuate within a specific range. If the wiring checks out, the sensor itself likely needs replacement. The A/F sensor is located on or near the exhaust manifold and can typically be replaced with an oxygen sensor socket and basic hand tools.
Severity
Symptoms
- •Check Engine light illuminated
- •Poor fuel economy
- •Rough idle or engine surging
- •Hesitation during acceleration
- •Engine may run rich
- •Failed emissions test
Likely Causes
Internal failure of the A/F sensor causes the AFS+ terminal to output abnormally low voltage, sending incorrect data to the ECM.
The wire carrying the AFS+ signal can become corroded, frayed, or shorted to ground, resulting in low voltage at the ECM terminal.
A loose, oxidized, or water-damaged connector at the A/F sensor prevents proper signal transmission, causing voltage dropout.
In rare cases, the ECM's input circuit for the A/F sensor reference voltage can fail, though this is uncommon.
Estimated Cost
Professional Repair
Includes parts + labor
Common Fixes
- Replace the air/fuel ratio sensor (sensor 1)
- Repair or replace damaged wiring in the A/F sensor circuit
- Clean and secure corroded electrical connectors
- Inspect for short circuits in the sensor harness