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2005 Chevy Optra P0128 and P0420 Trouble Codes: Diagnosis and Solutions

Model: 2005 Chevy Optra Fault Code: P0128 Posted: 2017-12-15 09:35

Hello everyone, I'm currently dealing with two trouble codes on my 2005 Chevrolet Optra: P0128 and P0420. I've been trying to resolve these issues myself and would greatly appreciate your expert advice. I initially tried a DIY solution using Dura Lube severe catalytic and exhaust treatment, switched from 87 to 91 octane fuel, and ran the vehicle for 200 kilometers. This temporarily cleared both codes—but they returned after about a week. Notably, my engine light is not on. I've observed a noticeable improvement in engine performance when switching to 91 octane gasoline. I'm planning an emissions test soon and would like guidance on the most effective next steps. I understand that P0128 typically points to a thermostat issue, which I plan to inspect or replace shortly. My main concern is the P0420 code—replacing the catalytic converter could cost significantly more than the vehicle's value. I'm also considering checking my oxygen (O2) sensors but don't know how to identify whether they're functioning properly or if they're faulty. As a last resort, I’m considering replacing the catalytic converter with an OEM part—but I’d like to avoid unnecessary expenses and ensure I’m making the right decision. Any insights from experienced users would be highly appreciated. Thanks in advance, monching

Related fault codes

P0128P0420

Comments (8)

Anonymous 2017-12-15 11:17

Prioritize fixing P0128 first—refer to this forum for common causes and solutions. Try Dura Lube again for P0420, especially if passing emissions is your goal. Check rockauto.com for current catalytic converter prices. Use a scan tool to read engine parameters. Verify fuel trims are within ±10%, ensure the engine reaches proper operating temperature (190–200°F), and confirm that O2 sensors are functioning normally.

Anonymous 2017-12-17 17:01

Thanks, cj1—very helpful! I've already ordered a thermostat from RockAuto and it's scheduled to arrive in early January. I added another bottle of Dura Lube, and the P0420 code has disappeared. The only remaining issue is now P0128. I'm hopeful it will stay gone. If needed, I'll try a third bottle before deciding whether to replace the catalytic converter.

Anonymous 2017-12-25 15:52

UPDATE: After a full tank refill, P0420 returned—this time showing only 'below threshold' without specifying bank 1. I’ve decided to wait for the thermostat replacement instead. The thermostat arrived on December 23 and was installed promptly. After bleeding the coolant system of air, both P0128 and P0420 have cleared. The engine now reaches proper operating temperature at the right time. I notice a significant improvement in power, responsiveness, and idle stability—no longer rough. While it's still early to confirm permanent resolution, I'm cautiously optimistic.

Anonymous 2017-12-29 20:35

Bad news—P0420 has returned again, but now only shows 'below threshold' without specifying bank 1. I’ve added another bottle of Dura Lube and will monitor the situation before making a decision on replacing the catalytic converter. I also came across a YouTube suggestion involving lacquer thinner—I’m considering trying it, but wanted to consult experienced forum members first for feedback. Any input would be greatly appreciated.

Anonymous 2017-12-29 23:29

If you're aiming for permanent code clearance, avoid relying solely on cleaners—they often don’t resolve the root cause and can come with a cost. What is the most affordable replacement catalytic converter available in your area? Regarding lacquer thinner—while it may be worth a try, it's not a guaranteed solution.

Anonymous 2017-12-30 07:12

The cheapest option I've found online is $120 per catalytic converter (front and rear), but these are aftermarket parts. Based on user reviews, such replacements may not last long. OEM parts typically cost around $700 for both converters, though they offer better longevity.

Anonymous 2017-12-30 09:13

Consider clearing the code with a cleaner before your emissions test—this might help avoid triggering P0420 under high load conditions. If that happens, reduce engine load during testing. Note: the front catalytic converter is likely responsible for the error. A budget option around $150 per converter may last several years and could be worth trying. If your O2 sensors are original, it might be worthwhile to replace them first before considering a full catalytic converter replacement.

Anonymous 2017-12-30 13:30

I'll follow that advice—thanks for the helpful input! Much appreciated.