Fault Tree

OBD-II differential diagnosis

A trouble code is a symptom report from one sensor, not a diagnosis, but it gets read as one. P0420 says catalyst efficiency below threshold and the driver is quoted $1,400.00 for a converter, when the cause is more often a $118.00 oxygen sensor. This takes a code plus the freeze frame the ECU captured when it set, and produces a ranked differential with prices attached.

Scans 5 vehicles

P0301
2018 Chevrolet Equinox LT
Cylinder 1 misfire detected
P0171
2014 Ford Escape SE
System too lean (bank 1)
P0455
2019 Toyota RAV4 LE
Evaporative emission system leak detected (large)
P0420
2016 Honda Accord LX
Catalyst system efficiency below threshold (bank 1)
P0128
2015 Subaru Outback 2.5i
Coolant thermostat below regulating temperature

Code structure decoder

SAE J2012 codes are structured, not arbitrary: the letter is the system, the first digit says whether the definition is generic or manufacturer-specific, and the third character selects the subsystem. Any well-formed code decodes even when no repair data is loaded for it.

Diagnosis

P0420 moderate will fail emissions
Catalyst system efficiency below threshold (bank 1)
The computer compares the oxygen sensor before the catalytic converter with the one after it. If both read the same, the converter is not doing its job, or one of the sensors is lying.
Vehicle
2016 Honda Accord LX · 2.4L I4 · 138,402 mi
Complaint
check engine light, no drivability complaint
Scanned
2026-07-22 04:35
Code family
Powertrain · generic SAE
Subsystem
Auxiliary emission controls

Most likely cause, given the freeze frame

Catalytic converter genuinely failed — 35% likely
Only after the four above are excluded. Confirm with a temperature differential across the converter body.
▸ O2 sensors within 0.03V of each other, converter is not storing oxygen
parts $1,180.00 + labour 2.2h @ $142.00/h = $1,492.40

Full ranked differential

CausePriorUpdatedRepair
Catalytic converter genuinely failed
O2 sensors within 0.03V of each other, converter is not storing oxygen
 21% 35% $1,492
Exhaust leak before or at the converter
O2 sensors within 0.03V of each other, converter is not storing oxygen
 19% 32% $244
Downstream O2 sensor (bank 1, sensor 2)
downstream sensor is responding, less likely to be the failed part
 30% 13% $217
Upstream O2 sensor lazy (bank 1, sensor 1) 16% 11% $231
Engine running rich (injector or fuel pressure) 14% 10% $142
The prior column is what the code alone tells you. The updated column is after conditioning on the freeze-frame values below. Green moved up on the evidence, red moved down.

What the diagnosis is worth

Work the ranked path, stop when fixed $1,785 expected
Replace catalytic converter genuinely failed first $1,492
Expected cost sums each cause's probability against the cumulative cost of reaching it in order. On this scan the ranked path expects $1,785 against $1,492 for going straight at the expensive component, which is the quote most people are handed.

Freeze frame — sensor state when the code set

PIDParameterValue
MAF Mass air flow rate 9.4 g/s
RPM Engine RPM 1840 rpm
SPEED Vehicle speed 72 km/h
O2_B1S1 O2 sensor, bank 1 sensor 1 0.61 V
O2_B1S2 O2 sensor, bank 1 sensor 2 0.58 V
LONG_FT1 Long-term fuel trim, bank 1 3.1 %
SHORT_FT1 Short-term fuel trim, bank 1 1.6 %
ENGINE_LOAD Calculated engine load 34 %
INTAKE_TEMP Intake air temperature 28 °C
COOLANT_TEMP Engine coolant temperature 89 °C