Code
P0422
Generic
P — Powertrain
Catalyst 2 Efficiency Below Threshold Bank 1
Views:
UK: 27
EN: 39
RU: 34
AI status
Completed
Completed
100%
Causes
- Degraded or clogged catalytic converter (Bank 1 downstream section)
- Faulty or slow downstream O2 sensor (Bank 1 sensor 2) or its heater circuit
- Exhaust leak upstream of the downstream O2 sensor or near the catalyst
- Fuel system problems causing rich or lean operation (injector fault, fuel pressure, vacuum leak)
- Repeated misfires or ignition problems contaminating/overheating catalyst
- Contaminants in catalyst (engine oil or coolant entering exhaust)
Symptoms
- Check Engine MIL illuminated (P0422 stored)
- Vehicle may fail emissions/inspection testing
- Reduced fuel economy if underlying fuel control issues exist
- Occasional rough running or misfire if related to engine issues
- Sulfur/rotten-egg smell from exhaust in severe catalyst contamination cases
- Usually no dramatic loss in driveability solely from P0422 unless accompanied by misfire or fuel issues
What to check
- Verify freeze frame and pending/full status with a scan tool; note engine conditions when code set
- Check for related DTCs (upstream/downstream O2 sensor codes, misfire codes, fuel trim codes)
- Visual inspection: damaged/dented exhaust, leaks, holes at gaskets/connections, melted catalyst housing, oil/coolant leaks into exhaust
- Monitor live data: upstream (B1S1) vs downstream (B1S2) O2 voltages and switching behavior during warm-up and steady cruise
- Compare short-term and long-term fuel trims (STFT/LTFT) for signs of rich/lean operation
- Check downstream O2 sensor heater resistance/current and wiring for opens/shorts/corrosion
Signal parameters
- Upstream O2 (Bank1 Sensor1) switching between ~0.1–0.9 V when closed-loop
- Downstream O2 (Bank1 Sensor2) should be relatively stable and not mirror upstream switching if catalyst is healthy; excessive switching indicates poor conversion
- Expected downstream O2 voltage ~0.4–0.6 V steady in closed-loop; rapid switching or large swings >0.2–0.7 V may indicate poor catalyst or bad sensor
- Short-term fuel trim (STFT) typically within ±10% in normal conditions
- Long-term fuel trim (LTFT) typically within ±10% (large positive/negative numbers indicate fuel control issues)
- Catalyst temperature delta (upstream vs downstream) often >100 °C under load if converter functioning; small delta suggests poor conversion
Diagnostic algorithm
- Retrieve and record freeze frame data and all related DTCs. Don’t assume P0422 alone pinpoints the catalyst.
- Clear codes, perform a proper drive cycle and re-check to confirm repeatability of P0422.
- Inspect exhaust system visually for leaks, damaged piping, missing gaskets or loose connections upstream of the downstream O2 sensor. Repair any leaks and retest.
- Using a scan tool, monitor B1S1 and B1S2 voltages: during warm, steady cruising B1S1 should switch and B1S2 should be relatively steady. If B1S2 mirrors B1S1, suspect catalyst inefficiency or a bad B1S2 sensor.
- Test downstream O2 sensor heater and signal wiring for continuity, shorts to ground/power and connector corrosion. Repair wiring as needed and retest for code persistence.
- Compare fuel trims and misfire counters. Diagnose root causes for high fuel trim or misfires (fuel pressure test, injector balance, compression/leak-down, ignition components) and repair before replacing catalyst.
- Perform a catalyst efficiency test: measure upstream and downstream exhaust temperatures with a pyrometer under controlled load (expect significant temperature drop across a working catalyst) or perform a differential pressure/backpressure test per service manual.
- If sensor wiring and engine operation are good, consider swapping a known-good downstream O2 sensor (or swap sensors if identical) to see if the code follows the sensor.
- If data and physical tests indicate poor catalyst performance, replace the catalytic converter and any damaged hardware. Use OEM or high-quality equivalent and ensure proper fitment and heat shielding.
- After any repair, clear codes and complete the required re-learn/drive cycle and/or reflash PCM if required. Confirm readiness monitors and verify P0422 does not return.
Likely causes
- Failed catalytic converter (Bank 1 substrate efficiency loss)
- Faulty Bank 1 downstream (sensor 2) oxygen sensor or heater
- Exhaust leak upstream of the downstream O2 sensor
- Fuel trim problem causing prolonged rich condition (injector, fuel pressure)
- Intermittent wiring/connector fault to downstream O2 sensor
Fault status
Status
Catalyst 2 efficiency below threshold (Bank 1). Downstream O2 readings indicate the catalytic converter (Bank 1) is not performing within expected efficiency. Inspect catalyst, downstream O2 sensor/wiring, exhaust leaks, and engine fuel/ignition issues.
Repair difficulty: Medium
Diagnostic time: 1-3 hours
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