I used to have both P1166 and P1167 error codes on my 2001 Honda Accord (F23A4 engine).

I fixed the P1166 by replacing the O2 sensor.

For about a day, everything looked wonderful (i.e. no check engine light on). Then, (after a day), the dreaded check engine light turned on again.

I used my OBDII device to check the code. It's P1167 now.

Googling this code raised the possibility of fuse #6 (15A) blown. I checked that fuse. It's 100% perfect.

What else could be the cause for this P1167 code?

  • Did you buy an OEM Honda part, or something from bosch/etc? It seems there's some part number problems as outline here: honda-acura.net/forums/accord/…
    – BReal14
    Nov 10, 2014 at 17:06
  • It is worth mentioning that I have the same engine with ultra-low emission. I guess ultra-low emission has air-fuel sensor and the wiring diagram is little different from others. The book says that both P1166 and p1167 is defined as the primary air/fuel sensor heater malfunction(not secondary).
    – KRoy
    Nov 3, 2017 at 6:46
  • Tengo un Honda Accord del 2000 código P 11 67 le cambie el sensor de arriba de oxígeno y el sensor de oxígeno de abajo chequien la cablería y todavía me sigue prendiendo el código que puede ser Dónde queda la ubicación del relay de oxígeno a ver si lo puedo conseguir no sé dónde queda Apr 24, 2020 at 2:02

4 Answers 4


The Air/Fuel Ratio (A/F) sensor 1 is installed in the exhaust system and detects oxygen content in the exhaust gas. The A/F sensor transmits output voltage to the Engine Control Module (ECM). A heater for the sensor element is embedded in the A/F sensor (sensor 1). If the A/F sensor (sensor 1) voltage is low, the air/fuel ratio is lean, and the ECM uses A/F feedback control to issue a Rich command. If the A/F sensor (sensor 1) voltage is high, the air/fuel ratio is rich, and the ECM uses A/F feedback control to issue a Lean command. The heater element is fused by fuse F6 as you say, but this fuse operates a relay. The relay may not be working. I dont have a diagram for this relay location, but it may be in your vehicles handbook. The check light went out possibly because you cancelled the code with your tester/scanner. Once the ECU ran its test on the A/F sensor the ECU found it faulty and turned it back on. Definitely the fault is on the heater side of the A/F sensor.

  • Belated thank you! How do I tell whether I need to replace the relay or to replace the A/F sensor? Also, what are the implications of that P1167 remaining ON for a long period? Does it damage anything in the car in anyway? Does it result in low milage? High emissions? Thanks.
    – ususer
    Dec 14, 2014 at 3:04
  • An Honda compatable scanner will read out your OBD2 codes, but will also add another two numbers to the end of the OBD2 code to pinpoint a fault. Dec 14, 2014 at 10:50

This is assuming you don't have access to a scantool that can communicate with a Honda ECM and without bidirectional controls.

Key On Engine Running (this may work Key On Engine off) disconnect the Air Fuel Ratio sensor connector. On the harness side connector Check for power on pin 4 (white). Check for ground on pin 3 (white/black).

If you don't have power on pin 4. Locate the Primary Heater Control Relay in the passenger under dash fuse box.

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Pull or backprobe the relay/connector.

Pin 30 (red/blue) - pulls power from fuse 6 of the passenger under dash fuse box.
Pin 87 (white) - is relay output to the ECM and AFR sensor.
Pin 86 (green/red) - is the relay coil control wire from the ECM.
Pin 87a - is unused and may be blank.
Pin 85 (black/yellow) - is the relay coil power, fuse 6 in the under hood fuse box.

If pins 30 or 85 don't have power check their respective fuses. If the fuses are OK, check continuity between the fuse terminals and the relay pins. Repair as necessary.

If you don't have ground on pin 86 backprobe pin 16 on the B connector at the ECM. If ground exists, check continuity on the circuit. Repair as necessary.

If you don't have power on pin 87 with the relay plugged in. Replace the relay.

If you don't have power on pin 4 of the AFR sensor connector, but have power on pin 87 of the relay. Check continuity between relay pin 87 and AFR sensor connector pin 4. Repair as necessary.

If pin 87 on the relay has power and pin 4 of the AFR sensor connector has power check for power at the ECM connector C pin 13 (white). If power exists and you have ground on pin 3 of the AFR sensor connector. Replace the AFR sensor first. If the code comes back replace the ECM.


P1166 indicates a blown or malfunctioning heater core fuse. I know that O2 sensors have resistive elements inside that are activated when the engine is cold. O2 sensors operate more efficiently when they are hot. The heater element is on until the engine is at normal operating temp then it switched off.

P1167 is the upstream O2 sensor just before the cat.

If it is anything like a evap code where the computer has to relearn the sensors parameters under varying load conditions by doing tests etc. Then after 3-5 hours down time depending on ambient temp it resets itself.

  • On a Honda the ECM monitors how many amps are pulled by the heater circuit with the heater relay commanded on. If drawn amps aren't in spec the ECM sets P1167 and or P1166.
    – Ben
    May 12, 2016 at 21:32
  • Why would the ECM monitor amps ? The ECM is essentially a computer that takes polls multiple inputs and derives the output based on that information. For example, when the engine temp is within a certain range yet the O2 sensor sends a value that doesn't conform to what the computer expects then it flags a check engine. These value ranges are all pre-programmed in memory.
    – Old_Fossil
    May 12, 2016 at 22:30
  • I'm no expert on how Honda ECM logic works. The way it's described the ECM monitors draw on the circuit in milliamps. If the circuit is open it doesn't draw anything. If resistance in the circuit is high it draws more than expected. This is almost exactly what you're describing. Except there's no feedback to the ECM on the heater circuit other than current. OTOH if you're talking about the actual o2 signal than yes there is feedback to the ECM. Both p1167 and 1166 are heater circuit codes.
    – Ben
    May 12, 2016 at 22:40
  • All the sensors outputs are current draws and produce a varying current and generate an analog signal. This signal is then digitally sampled and converted in discrete levels which are interpreted by the ECM as data .When all the info is analyzed the ECM responds accordingly.
    – Old_Fossil
    May 12, 2016 at 22:51
  • maybe i'm just being dumb, if i am, be patient. i don't see how a o2 heater element would return anything but current or resistance on the circuit. the mode $6 test for p1167 is, measure amount of amps drawn on the circuit. pass/fail
    – Ben
    May 12, 2016 at 23:14

As indicated in many of its reviews and Q&As, the Denso 234-9025 Air Fuel Ratio Sensor is a right fit for the F23A4 ULEV engine.

Amazon (as well as many part websites) might claim it is not compatible with your Honda Accord, since many part websites does not allow you to specify the specific engine code of your car. This causes such confusion.

The same part was also proposed in one of the comments (check that of "Boris the Spider") on the P1167 code explanation here.

I hope this answer helps future readers of this question.

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