OBD-II Diagnostic Trouble Code
P0420 — Catalyst System Efficiency Below Threshold (Bank 1)
P0420 means the ECM decided that the bank 1 catalyst is not storing and converting oxygen as well as it should, based on comparing the upstream and downstream oxygen sensors. On gas trucks it refers to the three-way catalytic converter; on some diesels the OEM uses it for diesel oxidation catalyst efficiency, judged by temperature rise or NOx sensor data instead of oxygen sensors.
Severity 2/5
Truck-relevant
Service soon
Severity
Service soon
Catalyst System Efficiency Below Threshold (Bank 1)
Code
P0420
Category
Powertrain
Subsystem
Emissions
Truck-relevant
Yes
What this means for truckers
P0420 (Catalyst System Efficiency) — see above.
Before you replace parts — try this first
P0420 on the Emissions system is most often worn or contaminated catalytic converter or doc — not a dead part. On commercial diesels especially, work these in order before spending money on parts:
- Confirm the symptoms. Match what your truck is doing to the Symptoms section below. If the code is set but the symptoms are not there, clear the code and re-test after a drive cycle — it may not return.
- Clean and dry the sensor connector. Water, road salt, or oil seeping into a connector is the #1 fake sensor failure. Disconnect, look for green corrosion or moisture, spray with electrical contact cleaner, dry, reseat. Many faults clear here for $0.
- Inspect the wiring harness. Trace the wire from the sensor toward the ECM — look for chafing against the engine block, melted insulation near the turbo or exhaust, or pinch points where a clamp has rubbed through.
- Test the sensor before swapping it. Most Common Causes (below) list the expected voltage or resistance range — a $20 multimeter saves a $300 sensor swap that does not fix the problem.
- Only then, replace the part. Use the OEM or a tier-1 aftermarket. Cheap sensors throw the same code in two weeks.
Symptoms
Usually only a check-engine light with no change in drivability. Failed emissions inspection is common. If the converter is physically plugged, you may notice low power and high exhaust temperature. On diesels using this code for the DOC, regens may take longer or fail.
Common causes
- CommonWorn or contaminated catalytic converter or DOC ($800–$3500)
- CommonUpstream engine problem such as misfire, oil burning or coolant leak that damaged the catalyst ($200–$4000)
- OccasionalExhaust leak ahead of or near the downstream oxygen sensor ($150–$600)
- OccasionalFaulty downstream or upstream oxygen sensor ($150–$450)
- RareECM software needing an update ($100–$300)
How to fix
Fix any misfire, fuel trim or engine oil or coolant consumption codes first
When to use: Always first
Check for exhaust leaks and compare upstream and downstream oxygen sensor signals
When to use: Before condemning the catalyst
Check for OEM software updates related to catalyst monitoring
When to use: When sensors and exhaust are good
Replace the catalytic converter or DOC with an emissions-compliant part
When to use: When tests confirm low catalyst efficiency
Affects most
- Chevrolet Silverado 35001 complaint mention this code
- Ford F-3501 complaint mention this code
- RAM 55001 complaint mention this code
Related
- → Browse all DTC trouble codes
- → Decode a VIN to check codes for your truck
- → Owner-operator maintenance guides
- → P0400 — Exhaust Gas Recirculation Flow Malfunction
- → P0401 — Exhaust Gas Recirculation Flow Insufficient Detected
- → P0402 — Exhaust Gas Recirculation Flow Excessive Detected
- → P0421 — Warm Up Catalyst Efficiency Below Threshold (Bank 1)
- → P0430 — Catalyst System Efficiency Below Threshold (Bank 2)
- → Complaints on Chevrolet Silverado 3500
- → Complaints on Ford F-350
- → Complaints on RAM 5500
Frequently Asked Questions
Can I drive with code P0420?
P0420 is severity 2 — service this week. Not an immediate stop, but ignoring it usually compounds the problem. Most common cause for this code is a worn sensor or minor performance fault that won't strand you today but will get more expensive the longer you wait.
How much does it cost to fix P0420 on a commercial truck?
Total repair cost ranges $100–$4000 depending on the root cause. The cheapest fix is usually the first one listed below — Worn or contaminated catalytic converter or DOC — which is also the most common. On commercial trucks the cost is usually higher than passenger vehicles due to part availability and shop labor rates ($120–185/hour at most truck shops, vs $90–130 at general auto shops). DIY-capable fixes (sensors, hoses, filters) are at the low end; major component replacements (turbo actuator, high-pressure fuel pump, transmission solenoid pack) are at the high end. Always diagnose with a scan tool BEFORE replacing parts — guessing wrong on a $1,500 turbo actuator when the actual issue was a $30 boost hose is a common expensive mistake.
Will P0420 fail a DOT inspection?
Yes — P0420 is an emissions-related code, and DOT inspectors check emissions compliance. An active emissions code with a check engine light can result in an out-of-service violation under FMCSA Part 393. The truck typically can't be moved until the code is cleared. Repeated emissions violations escalate to CSA scores.
What's the difference between P0420 and related codes?
P0420 is closely related to: P0400 (Exhaust Gas Recirculation Flow Malfunction), P0401 (Exhaust Gas Recirculation Flow Insufficient Detected), P0402 (Exhaust Gas Recirculation Flow Excessive Detected), P0421 (Warm Up Catalyst Efficiency Below Threshold (Bank 1)). These codes share the same subsystem (emissions) and often appear together. If you see this code, scan for the full code list — fixing the root cause usually clears multiple companion codes at once. If only P0420 is set without the companions, the issue is more isolated.
How do I prevent P0420 from coming back after a repair?
For P0420 (Catalyst System Efficiency Below Threshold (Bank 1)), prevention centers on the emissions system. Keep up with PM on the EGR system — clean the valve at 200K miles. Use OEM-spec air filters. Don't ignore minor codes; they often escalate into bigger emissions failures.