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Severity: Low Code type: Manufacturer-specific (Honda)

Honda P1157 Code: A/F Sensor (Sensor 1) AFS Circuit High Voltage

The signal lines of Honda's upstream air/fuel ratio sensor read too high, usually an open circuit.

At a glance

Severity: 2/5 – Low2/5

Severity: Low

Meaning
A/F Sensor (Sensor 1) AFS Circuit High Voltage
System
Fuel & Air Metering · Powertrain
Safe to drive?
Yes, short term
Typical repair cost
$0–$550
DIY difficulty
Easy

Many Hondas from the late 1990s onward, including Accords, CR-Vs, Elements and Civics, use a wideband air/fuel (A/F) sensor ahead of the catalytic converter instead of a simple oxygen sensor. The PCM holds a small bias voltage across the sensor's two signal lines, called AFS+ and AFS-, and reads the current the sensor pumps to work out the exact mixture. P1157 means the voltage on those lines went higher than the PCM accepts, which happens when the circuit is open: a broken element inside the sensor, a corroded or water-logged connector, or a broken wire. It's an electrical fault, so the fix is found with a meter rather than by chasing fuel trims.

Can I keep driving? Safe to drive in the short term. The PCM drops to a default fuel map, so mileage may dip and the car won't pass an emissions test until it's repaired.

How to read P1157

How to read P1157P11213574
  1. P Powertrain
  2. 1 Manufacturer-specific
  3. 1 Fuel & air metering
  4. 57 Specific fault

Symptoms

  • Check engine light on
  • Fuel economy a few mpg lower than usual
  • Slightly rough or rich-smelling exhaust after cold starts
  • A/F sensor reading frozen at one value in live data
  • Often no noticeable drivability change

Possible causes

Ranked by how often each cause is behind this code (editorial estimate).

  1. A/F sensor element or internal wiring failed open 45%
  2. Corroded, water-logged or grease-contaminated sensor connector 20%
  3. AFS wire broken or chafed where the harness crosses the exhaust manifold 15%
  4. Connector not fully latched after exhaust or engine work 8%
  5. Wrong or universal sensor fitted, or a poor splice 8%
  6. PCM A/F sensor input fault 4%

Diagnosis step by step

Diagnostic flow: P11571Confirm and note conditions2Inspect the A/F sensor connector3Check the AFS wires to the PCM4Watch live data on a warm engine5Replace the sensor correctly

  1. Confirm and note conditions

    Clear the code, start the engine and let it idle for a minute. If P1157 returns right away, the circuit is hard open; if it only returns after driving, suspect a connector or wire that opens with heat or vibration.

    Tools: OBD-II scanner

  2. Inspect the A/F sensor connector

    The 4-pin connector usually sits on a bracket near the exhaust manifold or at the back of the engine. Unplug it and look for water, green corrosion, melted grease or pushed-back pins, and check that the lock tab clicks when you reconnect it.

    Tools: Flashlight, Screwdriver

  3. Check the AFS wires to the PCM

    With the key off and the sensor unplugged, place a jumper between the two AFS pins on the harness side, then check for continuity between the two matching PCM terminals. No continuity means a broken wire; ground each AFS pin in turn to find which line is open.

    Tools: Multimeter

  4. Watch live data on a warm engine

    A working A/F sensor shows a lambda or equivalence ratio that moves around 1.00 at idle, goes clearly lean on a closed-throttle deceleration and rich on a hard throttle snap. A reading stuck at one value after the wiring checks out points to the sensor.

    Tools: OBD-II scanner

  5. Replace the sensor correctly

    Use the correct wideband A/F sensor for your engine, not a regular narrowband oxygen sensor, and apply anti-seize only to the threads. Tighten to about 32 lb-ft and route the pigtail away from the exhaust.

    Tools: O2 sensor socket, Torque wrench

Where the parts are

Exhaust: catalytic converter and O2 sensors (Bank 1 / Bank 2) – P11571212345
Exhaust: catalytic converter and O2 sensors (Bank 1 / Bank 2)
  1. 1Upstream O2 sensor
  2. 2Catalytic converter
  3. 3Downstream O2 sensor
  4. 4Diesel particulate filter
  5. 5Turbocharger

Repair options & costs

RepairDIY?Estimated cost
Clean and dry A/F sensor connector Yes $0–$80
Replace A/F sensor yourself (OE-type part) Yes $90–$250
Replace A/F sensor at a shop No $250–$550
Repair AFS wiring near the exhaust No $100–$300

Rough US estimates for parts and labor; shop rates vary by region. Not a quote.

Emissions / smog test impact

Will fail

With P1157 stored the check engine light is on, so the car fails an OBD emissions inspection. After a new sensor, a few drive cycles are needed for the O2 sensor and catalyst monitors to report ready.

Frequently asked questions

Is the Honda A/F sensor the same as an O2 sensor?

It sits in the same place but works differently: it measures the exact air/fuel ratio across a wide range instead of just switching rich or lean. That's why a generic narrowband oxygen sensor won't work as a replacement.

Will a cheap universal sensor fix P1157?

Often not. Honda's PCM is sensitive to the sensor's characteristics, and a mis-spliced universal sensor can trigger the same or new codes. A direct-fit Denso-type sensor with the factory plug is the safer choice.

Can water in the connector really cause this?

Yes. Water or corrosion between the AFS pins changes the tiny currents the PCM measures. Drying and cleaning the connector sometimes clears the code, but if the pins are green or the seal is gone, replace the connector or sensor.

What's the difference between P1157 and P1166?

P1157 is the signal side of the A/F sensor, the part that measures the mixture. P1166 and P1167 concern the heater inside the sensor that brings it to working temperature. Both can be caused by one failed sensor, but they're tested differently.

Sources