Code 420 catalytic converter keeps returning
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CF Beginner
Joined: Apr 2025
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From: Texas
Code 420 catalytic converter keeps returning
2018 colorado
have had the cat replaced 3 times by a reputable muffler shop, also the O2 sensors, and thermostat, had the engine flushed, replaced spark plugs and wires as well as some canister my dad replaced. The truck continues to throw the p0420 code (cat system low efficiency)& bank 1 below threshold.
when I say that both cats have been replaced 3 times no joke by muffler shop that warranted the work. But now they r saying that they can't warranty the work again because it has been too many times and it's not the parts must be the vehicle. I am wanting to know has anyone else had this issue and how to resolve.
have had the cat replaced 3 times by a reputable muffler shop, also the O2 sensors, and thermostat, had the engine flushed, replaced spark plugs and wires as well as some canister my dad replaced. The truck continues to throw the p0420 code (cat system low efficiency)& bank 1 below threshold.
when I say that both cats have been replaced 3 times no joke by muffler shop that warranted the work. But now they r saying that they can't warranty the work again because it has been too many times and it's not the parts must be the vehicle. I am wanting to know has anyone else had this issue and how to resolve.
CF Beginner
Joined: May 2025
Posts: 2
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2018 Colorado - Fixing P0420 Code for Good
Problem:
✔ Smoke test exhaust for leaks
✔ Check fuel trims with scan tool
✔ Look for oil/coolant in exhaust
✔ Try OEM O2 sensors (not aftermarket)
If Nothing Works:
For reliable parts, check Dynovox Parts for related products.
- Keeps throwing P0420 (bad cat code)
- Already replaced catalytic converters 3 times
- Also replaced O2 sensors, thermostat, plugs, wires, and canister
- Shop won't warranty another replacement
- Exhaust leaks before the cat (check with smoke test)
- Engine running rich (clogged injectors, bad fuel pressure)
- Oil/coolant burning (fouling the new cats)
- Bad O2 sensors (even new ones can be defective)
✔ Smoke test exhaust for leaks
✔ Check fuel trims with scan tool
✔ Look for oil/coolant in exhaust
✔ Try OEM O2 sensors (not aftermarket)
If Nothing Works:
- Try a high-quality aftermarket cat (like MagnaFlow)
- Have dealer check for needed software updates
For reliable parts, check Dynovox Parts for related products.
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