Reference meaning
What does this code mean?
The wording below belongs to the named make and communication. Match the complete code, reporting module, model year and engine before applying it.
A code identifies a detected condition, not a part that must automatically be replaced.
Narrow by vehicle
A listed vehicle is a research lead. Confirm the engine, production range and software in the manufacturer's information.
Before replacing a part
Record codes and freeze-frame data. Check battery condition, related codes and visible wiring or hose damage. Follow the vehicle's diagnostic procedure and test results before replacing parts.
Scan worksheet →Manufacturer wording
Source wording can vary by vehicle and suffix. It is shown as written, not as a universal diagnosis.
P0421Catalyst 1 Efficiency Below Threshold Bank 1
P0421-00Catalyst 1 Efficiency Below Threshold Bank 1
P0421Catalyst efficiency below threshold
Manufacturer communications
40Original wording in English
CHEVROLET, GMC
This technical bulletin provides repair information to correct a customer concern of Service Engine Soon Lamp being illuminated. They may also comment on multiple transmission issues including harsh or delayed shifts, slip, surge, shudder, noise, whine, growl, shake and/or vibration. As well as the exhaust brake not working or t…
RAM
Flash: Powertrain Control Module (PCM) Updates This bulletin involves reprogramming the Engine Control Module/Powertrain Control Module (ECM/PCM) with the latest available software. The customers may experience a Malfunction Indicator Lamp (MIL) illumination. Upon further investigation the technician may find one or more of the …
RAM
Flash: Powertrain Control Module (PCM) Updates This bulletin involves reprogramming the Engine Control Module/Powertrain Control Module (ECM/PCM) with the latest available software. The customers may experience a Malfunction Indicator Lamp (MIL) illumination. Upon further investigation the technician may find one or more of the …
CHEVROLET, GMC
This technical bulletin provides repair information to correct a customer concern of Service Engine Soon Lamp being illuminated with DTCs P2002, P2463, P2459, P144E, P0421, P0422, P0106 P20EE, P129F, P16F3, P20FF, P226D, P249D, P249E, P11CC and / or P11D5. They may also comment on multiple transmission issues including harsh or …
Check vehicle applicability
85A listed vehicle is a research lead. Confirm the engine, production range and software in the manufacturer's information.
| Make | Model | Model year |
|---|---|---|
| Audi | QUATTRO ↗ | 2001 |
| Chevrolet | CRUZE ↗ | 2017 |
| Chevrolet | EQUINOX ↗ | 2018 |
| Chevrolet | SILVERADO 2500 ↗ | 2020 |
| Chevrolet | SILVERADO 3500 ↗ | 2020 |
| Ford | PROBE ↗ | 1996 |
| GMC | SIERRA 2500 ↗ | 2020 |
| GMC | SIERRA 3500 ↗ | 2020 |
Code meaning and manufacturer information
Original wording in English · Historical directory labels; manufacturer applicability has not been established for every entry.
Warm Up Catalyst Efficiency Below Threshold (Bank 1)
Can I keep driving?
A code alone cannot establish whether driving is safe. A flashing malfunction light, severe shaking, overheating, oil-pressure warning or loss of braking or steering needs prompt attention. Stop in a safe place and seek assistance when control or engine safety is affected. Follow the vehicle handbook.