P0421 - Catalyst 1 Efficiency Below Threshold (Bank 1)
System
Powertrain (Engine & Gearbox)
Technical Description
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).
Multimeter Test Points & Nominal Values
| Test Point / Component | Measurement | Nominal Target Range | Test Condition |
|---|---|---|---|
| Catalyst 1 Efficiency Below Threshold (Bank 1) – Heater Resistance (Cold) | Resistance (Ω) | 2.5 - 10.0 Ω | 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 | 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) | Warm engine, closed loop |
Disconnected plug at 20°C
Warm engine closed loop at 2000 RPM
Warm engine, closed loop
Oscilloscope Waveform Analysis
1 s/div, 200 mV/divPossible Causes
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).
Chafed or pinched wiring harness along the signal path to Catalyst 1 Efficiency Below Threshold (Bank 1)
Poor ground connection or excessive voltage drop (> 0.1 V) at the ground point for Catalyst 1 Efficiency Below Threshold (Bank 1)
Defective internal output driver or reference circuit in the control module for Catalyst 1 Efficiency Below Threshold (Bank 1)
Symptoms
4
Recommended Solution
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's 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.