What this code means
P0420 usually means the ECM may be seeing low catalyst efficiency on Bank 1.
What the vehicle may do
- The vehicle may only show a check engine light.
- Driveability may seem normal, or symptoms may appear if an engine-running problem is contributing.
Possible fault areas
- Catalytic converter condition
- Exhaust leaks, restrictions, or damage
- Downstream oxygen sensor condition or wiring
- Possible engine conditions that can degrade a catalyst
Diagnostic path
What P0420 is telling us
On this 2014 to 2018 GMC Sierra 1500 with the 5.3 gas engine, P0420 means the ECM has judged catalyst efficiency low on Bank 1. In plain language, the catalyst may not be cleaning up the exhaust the way the ECM expects. The truck may only have a check engine light, or it may have other drivability clues if an engine-running issue is contributing. Possible areas can include the catalyst itself, exhaust leaks or restrictions, downstream oxygen sensor condition, wiring, or engine conditions that can damage a converter over time. For this code, stay focused on Bank 1, and let the scan tool monitor tell you whether the test passes, fails, or never completes.
Start with the basic checks
Before getting deep into the catalyst test, start with the basic system checks and follow a structured diagnostic approach. Turn the ignition ON and check the vehicle for other current codes. If other codes are present, check what they mean first instead of chasing the catalyst code in isolation. Catalyst damage can be caused by things like misfire, high oil or coolant consumption, retarded spark timing, weak spark, lean or rich fuel mixture, or oxygen sensor and harness problems, so those need to be considered before condemning the converter.
Reproduce the catalyst monitor conditions
Next, reproduce the operating conditions for the catalyst monitor. Treat these as monitor gates, not component test results. The catalyst diagnostic enables with engine speed between 1,000–2,500 RPM, and once it is in range, engine speed must stay between 950–2,550 RPM to keep the test enabled. Accelerator pedal position is less than 100 %. The post HO2S heaters are ON for greater than 60 s. MAF is between 4–20 g/s. System voltage is greater than 10 V. Vehicle speed is between 62–120 km/h (38.5–74.6 MPH) to enable the test, and between 58–125 km/h (36–77.7 MPH) to keep it enabled. Catalyst calculated temperature is between 600–900°C (1,112–1,652°F). Ethanol estimation and EVAP purge are not active, the engine is in closed loop, and the engine is operating in decel fuel cut off. The ECM attempts one test during each valid off-idle period after the gates are met, up to 3 tests in a drive cycle.
Use the scan tool monitor result
Watch the scan tool Catalyst Monitor Not At Idle Test Conditions Met parameter. You want it to change to Yes. If it does not show Yes, pause and re-establish the operating conditions instead of guessing at the result. Once it shows Yes, monitor Catalyst Monitor Test State. When it says Active, the catalyst monitor is running. Keep watching until it goes Inactive, because that means the test has completed. At completion, the Catalyst Monitor Test Counter for the banks increments by one, and the Catalyst Monitor Test Result will show Passed, Failed, or No Result. For P0420, pay attention to the Bank 1 result. If the result is No Result, repeat the monitor-running process until it reports Passed or Failed. If Bank 1 passes, the system is OK at that point. If it fails, move into the physical testing path.
Physical testing after a failed monitor
If the monitor fails, inspect the catalytic converters for dents, severe discoloration from excessive temperature, road damage, internal rattles from damaged catalyst substrate, and restrictions. If one of those converter conditions is found, replace the catalytic converters. If the converters do not show those conditions, inspect the exhaust system for leaks, restrictions, physical damage, and loose or missing hardware. If an exhaust condition is found, repair or replace the affected component as necessary. If the exhaust system checks out, inspect the appropriate downstream heated oxygen sensor, the B52B heated oxygen sensor 2, for incorrect torque or damage. If one of those sensor conditions is found, replace that sensor or repair the condition as necessary. If none of those conditions are found, the remaining path is to test or replace the catalytic converters.
After the repair
After any repair, handle the setup work before verification. If a heated oxygen sensor was replaced, perform the HO2S Heater Learn procedure with the scan tool. Reinstall any components removed during diagnosis, and perform any required adjustments, programming, or setup procedures for the parts that were removed or replaced. Then clear the DTCs with the scan tool and turn the ignition OFF for 60 s.
Verify the repair
Keep verification separate from testing. After clearing the code and waiting with the ignition OFF for 60 s, operate the vehicle within the same catalyst monitor conditions. Confirm the Catalyst Monitor Not At Idle Test Conditions Met parameter changes to Yes. If it does not, re-establish the operating conditions. When the Catalyst Monitor Test State goes Active, let the monitor run until it goes Inactive. The test counter should increment, and the result should report Passed, Failed, or No Result. If the result is No Result, repeat the verification drive until it reports Passed or Failed. If the result fails, go back to the physical testing path. If Bank 1 passes, the repair is verified and the code stayed gone. Takeaway: don’t condemn the converter until the monitor has actually run, the result is known, and the exhaust and downstream sensor checks have been worked in order. For more diagnostic training, visit stepdiagnostics.com.
Final check
P0420 should be diagnosed by confirming the catalyst monitor result and then checking the converter, exhaust system, and downstream oxygen sensor path in order.
For more guided automotive diagnostics, visit STEP Diagnostics.
