{
 "id": "p0420-accord-o2",
 "title": "Front and rear oxygen sensors on a 2010 Honda Accord 2.4 with P0420 stored",
 "recorded": "2026-09-28",
 "car": {
  "year": 2010,
  "make": "Honda",
  "model": "Accord",
  "engine": "2.4 L four-cylinder",
  "mileage": null
 },
 "codes_at_drive_start": {
  "stored": [
   "P0420",
   "P0455",
   "P0456"
  ],
  "pending": [],
  "permanent": [],
  "check_engine_light": "off",
  "readiness": "all supported monitors complete, catalyst included",
  "note": "Read with Mode 03, 07, 0A and PID 01 at the start of the drive."
 },
 "sensors": {
  "front": {
   "label": "Front sensor (Bank 1 Sensor 1)",
   "kind": "wide-band air-fuel sensor",
   "unit": "lambda (1.00 = ideal mixture)",
   "pid": "Mode 01 PID 34 (equivalence ratio and current)"
  },
  "rear": {
   "label": "Rear sensor (Bank 1 Sensor 2)",
   "kind": "narrow-band oxygen sensor",
   "unit": "volts",
   "pid": "Mode 01 PID 15"
  }
 },
 "window": {
  "definition": "From the first second coolant read 75 C (167 F) to the first decel fuel cut.",
  "start_s_into_drive": 116,
  "end_s_into_drive": 279,
  "seconds": 163,
  "fuel_system": "closed loop for every reading in the window",
  "fuel_cut_in_window": false
 },
 "method": {
  "sampling": "About 1 reading per second per sensor. Timestamps are whole seconds.",
  "dedupe": "Exact duplicate rows dropped; one reading per sensor per second kept (the last).",
  "crossing": "A crossing is the rear voltage moving from one side of 0.45 V to the other between two kept readings.",
  "other_counts": "Counted another way, the same drive gives more: 24 crossings when every raw reading is kept, 22 when the first reading in each second is kept. We show the lowest count."
 },
 "stats": {
  "rear_crossings_0_45V": 18,
  "rear_crossing_times_s": [
   22,
   24,
   26,
   29,
   39,
   40,
   49,
   62,
   73,
   74,
   82,
   83,
   100,
   101,
   127,
   128,
   148,
   153
  ],
  "rear_v_min": 0.035,
  "rear_v_max": 0.805,
  "rear_v_mean": 0.59,
  "rear_share_below_0_45V": 0.166,
  "front_lambda_min": 0.794,
  "front_lambda_max": 1.094,
  "long_term_fuel_trim_pct_range": [
   -18.0,
   -16.4
  ],
  "speed_kmh_max": 44.0,
  "speed_mph_max": 27,
  "rear_points": 151,
  "front_points": 155
 },
 "captions": {
  "headline": "On our own 2010 Accord with P0420 stored, the rear sensor crossed 0.45 V 18 times in 163 warm seconds, with no fuel cut.",
  "short": [
   "This is our own 2010 Honda Accord 2.4. P0420 was stored in the engine computer when we plugged in.",
   "Grey line: the front sensor. On this engine it is a wide-band air-fuel sensor, so it reads in lambda, not volts. 1.00 is the ideal mixture.",
   "Amber line: the rear sensor, after the catalytic converter. It crossed 0.45 V 18 times in 163 seconds once the engine was warm.",
   "The engine stayed in closed loop the whole time. There was no fuel cut, so the drops are not the injectors switching off.",
   "A healthy converter stores oxygen, so its rear sensor usually changes slowly. Here it keeps dropping to the low end and back."
  ],
  "alt": "Two line charts over 163 seconds of warm driving. Top: front air-fuel sensor, lambda between 0.79 and 1.09, mostly near 1.00. Bottom: rear oxygen sensor voltage between 0.04 and 0.81 V, crossing the 0.45 V line 18 times.",
  "caveats": [
   "The app read each sensor about once per second. A sensor can switch faster than that, so the real number of crossings can be higher. Treat the count as a floor.",
   "Some of the drops came while coasting with the throttle closed. A lean exhaust while coasting is normal, and a long coast can pull even a good converter's rear sensor low.",
   "One short drive (about 1.4 km) is not a diagnosis. The engine computer's own catalyst test is what stored P0420; this chart shows what that looks like from the outside.",
   "Long-term fuel trim held between -18% and -16% for the whole window: the computer was taking fuel away. We have not found the cause yet. Rich running is one known way converters get damaged, so it is worth checking before a new converter goes in.",
   "The check engine light was off during this drive, with P0420 still stored. Stored codes can stay in memory after the light goes out."
  ]
 },
 "series": {
  "t_s": "seconds since the start of the warm window",
  "front_lambda": [
   [
    0,
    1.038
   ],
   [
    1,
    1.016
   ],
   [
    2,
    0.993
   ],
   [
    3,
    1.004
   ],
   [
    4,
    0.993
   ],
   [
    5,
    0.982
   ],
   [
    6,
    0.992
   ],
   [
    7,
    1.006
   ],
   [
    8,
    1.006
   ],
   [
    9,
    0.999
   ],
   [
    10,
    1.0
   ],
   [
    11,
    1.008
   ],
   [
    12,
    1.005
   ],
   [
    13,
    0.971
   ],
   [
    14,
    0.961
   ],
   [
    15,
    0.966
   ],
   [
    16,
    0.996
   ],
   [
    17,
    0.976
   ],
   [
    19,
    1.005
   ],
   [
    20,
    0.999
   ],
   [
    21,
    0.989
   ],
   [
    22,
    0.991
   ],
   [
    23,
    1.013
   ],
   [
    24,
    0.985
   ],
   [
    25,
    0.902
   ],
   [
    26,
    0.958
   ],
   [
    27,
    1.094
   ],
   [
    28,
    0.981
   ],
   [
    29,
    0.975
   ],
   [
    30,
    0.974
   ],
   [
    31,
    0.987
   ],
   [
    33,
    1.017
   ],
   [
    34,
    0.993
   ],
   [
    35,
    0.993
   ],
   [
    36,
    1.0
   ],
   [
    37,
    1.009
   ],
   [
    38,
    1.018
   ],
   [
    39,
    0.99
   ],
   [
    40,
    0.979
   ],
   [
    41,
    0.992
   ],
   [
    42,
    0.985
   ],
   [
    43,
    0.998
   ],
   [
    45,
    0.995
   ],
   [
    46,
    0.95
   ],
   [
    47,
    0.794
   ],
   [
    49,
    1.09
   ],
   [
    50,
    1.014
   ],
   [
    51,
    0.996
   ],
   [
    53,
    1.005
   ],
   [
    55,
    0.985
   ],
   [
    56,
    0.986
   ],
   [
    57,
    0.993
   ],
   [
    58,
    0.997
   ],
   [
    59,
    0.995
   ],
   [
    60,
    1.016
   ],
   [
    61,
    1.014
   ],
   [
    62,
    0.989
   ],
   [
    63,
    0.991
   ],
   [
    64,
    0.993
   ],
   [
    65,
    0.978
   ],
   [
    66,
    1.015
   ],
   [
    67,
    0.965
   ],
   [
    68,
    1.031
   ],
   [
    69,
    0.992
   ],
   [
    70,
    1.031
   ],
   [
    71,
    0.971
   ],
   [
    72,
    1.022
   ],
   [
    73,
    1.031
   ],
   [
    74,
    0.967
   ],
   [
    75,
    0.992
   ],
   [
    76,
    0.977
   ],
   [
    77,
    1.056
   ],
   [
    78,
    0.995
   ],
   [
    79,
    0.995
   ],
   [
    80,
    0.891
   ],
   [
    81,
    0.893
   ],
   [
    82,
    0.999
   ],
   [
    84,
    0.993
   ],
   [
    85,
    0.994
   ],
   [
    86,
    0.988
   ],
   [
    87,
    0.941
   ],
   [
    88,
    1.042
   ],
   [
    90,
    1.013
   ],
   [
    91,
    0.996
   ],
   [
    92,
    0.993
   ],
   [
    93,
    0.996
   ],
   [
    94,
    1.016
   ],
   [
    95,
    1.01
   ],
   [
    96,
    0.993
   ],
   [
    97,
    1.0
   ],
   [
    98,
    1.016
   ],
   [
    99,
    1.012
   ],
   [
    100,
    1.039
   ],
   [
    101,
    0.986
   ],
   [
    102,
    0.967
   ],
   [
    103,
    1.02
   ],
   [
    104,
    1.012
   ],
   [
    105,
    0.998
   ],
   [
    106,
    0.982
   ],
   [
    107,
    0.997
   ],
   [
    108,
    1.001
   ],
   [
    109,
    1.011
   ],
   [
    110,
    0.998
   ],
   [
    111,
    1.004
   ],
   [
    112,
    1.012
   ],
   [
    113,
    1.007
   ],
   [
    114,
    1.004
   ],
   [
    115,
    1.0
   ],
   [
    116,
    1.001
   ],
   [
    117,
    0.998
   ],
   [
    118,
    1.009
   ],
   [
    119,
    0.999
   ],
   [
    120,
    1.0
   ],
   [
    121,
    1.01
   ],
   [
    122,
    0.996
   ],
   [
    123,
    1.011
   ],
   [
    124,
    1.02
   ],
   [
    125,
    0.94
   ],
   [
    126,
    0.902
   ],
   [
    127,
    1.023
   ],
   [
    128,
    0.965
   ],
   [
    129,
    0.999
   ],
   [
    130,
    1.005
   ],
   [
    131,
    0.984
   ],
   [
    132,
    1.005
   ],
   [
    133,
    1.006
   ],
   [
    134,
    0.971
   ],
   [
    135,
    0.971
   ],
   [
    136,
    1.008
   ],
   [
    137,
    0.987
   ],
   [
    138,
    0.98
   ],
   [
    139,
    0.989
   ],
   [
    140,
    1.008
   ],
   [
    141,
    0.991
   ],
   [
    142,
    1.005
   ],
   [
    143,
    0.961
   ],
   [
    144,
    1.024
   ],
   [
    145,
    1.002
   ],
   [
    146,
    1.034
   ],
   [
    147,
    1.015
   ],
   [
    148,
    1.008
   ],
   [
    149,
    0.969
   ],
   [
    150,
    1.013
   ],
   [
    151,
    1.002
   ],
   [
    152,
    0.976
   ],
   [
    153,
    0.967
   ],
   [
    154,
    0.95
   ],
   [
    155,
    0.988
   ],
   [
    156,
    0.988
   ],
   [
    157,
    1.001
   ],
   [
    158,
    0.985
   ],
   [
    159,
    0.996
   ],
   [
    160,
    0.998
   ],
   [
    161,
    0.993
   ],
   [
    162,
    0.991
   ]
  ],
  "rear_v": [
   [
    0,
    0.685
   ],
   [
    1,
    0.64
   ],
   [
    2,
    0.645
   ],
   [
    3,
    0.64
   ],
   [
    4,
    0.635
   ],
   [
    5,
    0.64
   ],
   [
    6,
    0.64
   ],
   [
    7,
    0.64
   ],
   [
    9,
    0.66
   ],
   [
    10,
    0.67
   ],
   [
    11,
    0.655
   ],
   [
    12,
    0.655
   ],
   [
    13,
    0.625
   ],
   [
    14,
    0.76
   ],
   [
    15,
    0.735
   ],
   [
    16,
    0.735
   ],
   [
    17,
    0.74
   ],
   [
    18,
    0.71
   ],
   [
    19,
    0.605
   ],
   [
    20,
    0.515
   ],
   [
    21,
    0.69
   ],
   [
    22,
    0.1
   ],
   [
    23,
    0.24
   ],
   [
    24,
    0.71
   ],
   [
    25,
    0.795
   ],
   [
    26,
    0.12
   ],
   [
    27,
    0.035
   ],
   [
    28,
    0.195
   ],
   [
    29,
    0.7
   ],
   [
    30,
    0.74
   ],
   [
    31,
    0.76
   ],
   [
    33,
    0.67
   ],
   [
    34,
    0.64
   ],
   [
    35,
    0.635
   ],
   [
    36,
    0.62
   ],
   [
    37,
    0.62
   ],
   [
    38,
    0.64
   ],
   [
    39,
    0.08
   ],
   [
    40,
    0.645
   ],
   [
    41,
    0.665
   ],
   [
    42,
    0.735
   ],
   [
    43,
    0.7
   ],
   [
    45,
    0.69
   ],
   [
    46,
    0.75
   ],
   [
    47,
    0.805
   ],
   [
    49,
    0.04
   ],
   [
    51,
    0.1
   ],
   [
    53,
    0.14
   ],
   [
    55,
    0.13
   ],
   [
    56,
    0.135
   ],
   [
    57,
    0.15
   ],
   [
    58,
    0.16
   ],
   [
    59,
    0.17
   ],
   [
    60,
    0.19
   ],
   [
    61,
    0.375
   ],
   [
    62,
    0.645
   ],
   [
    63,
    0.72
   ],
   [
    64,
    0.67
   ],
   [
    65,
    0.735
   ],
   [
    66,
    0.655
   ],
   [
    67,
    0.75
   ],
   [
    68,
    0.635
   ],
   [
    69,
    0.625
   ],
   [
    71,
    0.7
   ],
   [
    72,
    0.65
   ],
   [
    73,
    0.085
   ],
   [
    74,
    0.745
   ],
   [
    75,
    0.725
   ],
   [
    76,
    0.74
   ],
   [
    77,
    0.665
   ],
   [
    78,
    0.67
   ],
   [
    79,
    0.595
   ],
   [
    80,
    0.59
   ],
   [
    81,
    0.79
   ],
   [
    82,
    0.115
   ],
   [
    83,
    0.56
   ],
   [
    84,
    0.63
   ],
   [
    85,
    0.655
   ],
   [
    86,
    0.69
   ],
   [
    87,
    0.735
   ],
   [
    88,
    0.715
   ],
   [
    90,
    0.645
   ],
   [
    91,
    0.63
   ],
   [
    92,
    0.62
   ],
   [
    94,
    0.555
   ],
   [
    95,
    0.53
   ],
   [
    96,
    0.605
   ],
   [
    97,
    0.475
   ],
   [
    98,
    0.67
   ],
   [
    99,
    0.64
   ],
   [
    100,
    0.435
   ],
   [
    101,
    0.635
   ],
   [
    102,
    0.675
   ],
   [
    103,
    0.635
   ],
   [
    104,
    0.62
   ],
   [
    105,
    0.625
   ],
   [
    106,
    0.69
   ],
   [
    107,
    0.69
   ],
   [
    108,
    0.67
   ],
   [
    109,
    0.675
   ],
   [
    110,
    0.68
   ],
   [
    111,
    0.665
   ],
   [
    112,
    0.655
   ],
   [
    113,
    0.635
   ],
   [
    114,
    0.65
   ],
   [
    115,
    0.665
   ],
   [
    116,
    0.65
   ],
   [
    117,
    0.645
   ],
   [
    118,
    0.63
   ],
   [
    119,
    0.64
   ],
   [
    120,
    0.62
   ],
   [
    121,
    0.65
   ],
   [
    122,
    0.63
   ],
   [
    123,
    0.625
   ],
   [
    124,
    0.575
   ],
   [
    125,
    0.735
   ],
   [
    126,
    0.775
   ],
   [
    127,
    0.265
   ],
   [
    128,
    0.745
   ],
   [
    129,
    0.685
   ],
   [
    130,
    0.69
   ],
   [
    131,
    0.725
   ],
   [
    132,
    0.725
   ],
   [
    133,
    0.68
   ],
   [
    134,
    0.725
   ],
   [
    135,
    0.675
   ],
   [
    136,
    0.645
   ],
   [
    137,
    0.69
   ],
   [
    138,
    0.675
   ],
   [
    139,
    0.655
   ],
   [
    140,
    0.655
   ],
   [
    141,
    0.685
   ],
   [
    142,
    0.655
   ],
   [
    143,
    0.725
   ],
   [
    144,
    0.665
   ],
   [
    145,
    0.665
   ],
   [
    146,
    0.515
   ],
   [
    147,
    0.665
   ],
   [
    148,
    0.355
   ],
   [
    149,
    0.355
   ],
   [
    150,
    0.41
   ],
   [
    151,
    0.135
   ],
   [
    152,
    0.23
   ],
   [
    153,
    0.62
   ],
   [
    154,
    0.75
   ],
   [
    156,
    0.73
   ],
   [
    158,
    0.73
   ],
   [
    159,
    0.705
   ],
   [
    160,
    0.7
   ],
   [
    161,
    0.71
   ],
   [
    162,
    0.73
   ]
  ]
 },
 "mode06": {
  "note": "Mode 06 results the app saved on this car on earlier scans (2026-08-14 to 2026-09-24). The 2026-09-28 drive did not save Mode 06.",
  "catalyst_bank1": {
   "test": "MID 0x21, TID 0xA1. Test ids 0x80 and up are defined by the carmaker, not by the OBD standard, so we read it only as pass or fail against its own limit.",
   "scans_with_a_result": 12,
   "first": "2026-09-21",
   "last": "2026-09-24",
   "value": [
    2.785
   ],
   "limit_max": [
    3.0
   ],
   "passed": true,
   "caption": "On every scan from 2026-09-21 to 2026-09-24, the stored result of the car's own catalyst test was 2.785 against a limit of 3.0: a pass, but close to the limit. A converter near its limit can pass on some drives and fail on others, which is one reason P0420 comes and goes."
  },
  "misfire_last_cycle_by_cylinder": {
   "1": {
    "scans": 17,
    "max": 3.0
   },
   "2": {
    "scans": 18,
    "max": 0.0
   },
   "3": {
    "scans": 18,
    "max": 0.0
   },
   "4": {
    "scans": 17,
    "max": 0.0
   }
  }
 }
}