What does fault code P0421 mean?

The control module has detected a fault in the Catalyst 1 Efficiency Below Threshold (Bank 1) (P0421). Affects cylinder bank 1 (bank 1 containing cylinder 1, firewall/right side). Measurements: • Catalyst 1 Efficiency Below Threshold (Bank 1) – Heater Resistance (Cold) (Resistance (Ω)): 2.5 - 10.0 Ω [Condition: Disconnected plug at 20°C] • Catalyst 1 Efficiency Below Threshold (Bank 1) – Signal Voltage (Voltage (V)): 0.10 - 0.90 V, oscillating 1 - 3 Hz [Condition: Warm engine closed loop at 2000 RPM] • Catalyst 1 Efficiency Below Threshold (Bank 1) – Pump Current (Wideband) (Current (mA)): -2.0 to +2.0 mA (0 mA at lambda 1.0) [Condition: Warm engine, closed loop] Oscilloscope: Scope the signal on a warm engine at 2000 RPM. A healthy Catalyst 1 Efficiency Below Threshold (Bank 1) swings between 0.1 V and 0.9 V at least 8 times in 10 seconds; a lazy or flat trace indicates sensor aging or carbon buildup. (Scale: 1 s/div, 200 mV/div)

What are the symptoms of fault code P0421?

Subsystem operating efficiency falls below calibrated threshold Check Engine Light (MIL) illuminated Possible drivability or power reduction Risk of failing an emissions inspection

What causes fault code P0421?

Most likely: Catalyst 1 Efficiency Below Threshold (Bank 1) is electrically operational but performing outside calibrated tolerance Bank 1 specific: Inspect wiring harness to cylinder bank 1 (firewall/right bank, exposed to exhaust manifold heat). Likely: Chafed or pinched wiring harness along the signal path to Catalyst 1 Efficiency Below Threshold (Bank 1) Likely: Poor ground connection or excessive voltage drop (> 0.1 V) at the ground point for Catalyst 1 Efficiency Below Threshold (Bank 1) Rare: Defective internal output driver or reference circuit in the control module for Catalyst 1 Efficiency Below Threshold (Bank 1)

How do you fix fault code P0421?

Cross-swap the component on Bank 1 with Bank 2 to rule out mechanical oil passages and internal timing wear. Verify underlying mechanical operation and rule out blockage, vacuum leaks, or mechanical binding. Verify loaded supply voltage and harness terminal pins at Catalyst 1 Efficiency Below Threshold (Bank 1). Check circuit continuity and ground-side voltage drop (< 0.05 V) for Catalyst 1 Efficiency Below Threshold (Bank 1). For wideband sensors: monitor pump current (mA) and lambda in scan Live Data during snap-throttle and decel fuel cut. Mechanic tip: O2 sensors measure free oxygen, not fuel. An exhaust leak upstream of the sensor pulls in atmospheric air via venturi pulses, forcing false lean readings and causing the ECM to severely over-fuel.

Standard EOBD

P0421 - Katalysator 1 – renseeffekt under terskelverdi (Bank 1)

Relatert:Katalysator 1, Ledningsnett og kontakter, Styreenhet (ECM/PCM)

System

Drivlinje (Motor og girkasse)

Kontakt oss Finn reparasjon

Teknisk beskrivelse

Styreenheten registrerer en feil i Katalysator 1 (P0421). Gjelder sylinderbank 1 (bank 1 med sylinder 1).

Multimeter Målepunkter & Nominelle Verdier

Målepunkt / Komponent	Måletype	Nominelt Målområde	Målevilkår
Katalysator 1 – varmemotstand (kald)	Motstand (Ω)	2.5 - 10.0 Ω	Frakoblet ved 20°C
Katalysator 1 – signalspenning	Spenning (V)	0.10 - 0.90 V, oscillerande 1 - 3 Hz	Varm motor i sluten loop vid 2000 RPM

Katalysator 1 – varmemotstand (kald)Motstand (Ω)

Børverdi:2.5 - 10.0 Ω

Frakoblet ved 20°C

Katalysator 1 – signalspenningSpenning (V)

Børverdi:0.10 - 0.90 V, oscillerande 1 - 3 Hz

Varm motor i sluten loop vid 2000 RPM

Oscilloskop Kurveformanalyse

1 s/div, 200 mV/div

Mål signalspenning for Katalysator 1 ved 2000 o/min; veksler mellom 0.1 V og 0.9 V.

Mulige årsaker

Katalysator 1 fungerer elektrisk, men yter utenfor toleranse

Bank 1:Kontroller ledningsnettet til sylinderbank 1 (mot torpedovegg).

Gnagd eller klemt ledningsnett fram til Katalysator 1

Dårlig jordforbindelse eller spenningsfall (> 0.1 V) for Katalysator 1

Defekt driver eller referanseutgang i styreenheten for Katalysator 1

Symptomer

Systemets virkningsgrad faller under kalibrert grense

Motorlampe (MIL) lyser

Mulig redusert kjørbarhet eller effekt

Risiko for underkjent EU-kontroll

Anbefalt tiltak

Bytt om komponenten mellom Bank 1 og Bank 2 for å bekrefte feilen før ny del bestilles.

Sjekk mekanisk grunnfunksjon og utelukk lekkasjer eller tette kanaler.

Sjekk matespenning og kontaktstykke ved Katalysator 1.

Mål kontinuitet og jordspenningsfall (< 0.05 V) for Katalysator 1.

For bredbåndssonde:sjekk pumpestrøm (mA) og lambdaverdier i livedata.

Mekanikerens tips

Eksoslekkasje før lambdasonden suger inn falskluft og lurer styreenheten til å fete opp blandingen unødig.

Relaterte feilkoder

P046D

Eksosresirkulering (EGR) sensor 'A'' – periodisk / ustabil signal

P040D

Eksosresirkulering (EGR) temperatursensor 'A' – høyt signal

P0494

Fan turtall/hastighet – lavt signal

P0497

Fordampingsutslipp (EVAP) for lav luftestrøm

Målepunkt / Komponent

Måletype

Nominelt Målområde

Målevilkår

Katalysator 1 – varmemotstand (kald)

Motstand (Ω)

2.5 - 10.0 Ω

Frakoblet ved 20°C

Katalysator 1 – signalspenning

Spenning (V)

0.10 - 0.90 V, oscillerande 1 - 3 Hz

Varm motor i sluten loop vid 2000 RPM