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2003 Ford Fiesta 1.4 Zetec P0420 Code: Misfire & Catalytic Converter Efficiency Explained

Model: Fiesta Fault Code: P0420 Posted: 2010-07-01 17:02

My 2003 Ford Fiesta 1.4 Zetec petrol engine had a coil pack failure affecting two cylinders, which I replaced recently. After the repair, the engine runs smoothly, but the check engine light remains on. Using an OBD2 code reader, I detected fault code P0420 — indicating reduced catalytic converter efficiency. Could the previous misfire have damaged the converter? I'm concerned about whether replacing just the coil pack was enough to resolve this issue. Leefish

Related fault codes
P0420
Comments (8)
Anonymous 2010-07-01 18:50

A prolonged misfire can flood the catalytic converter with unburnt fuel, potentially leading to reduced efficiency and triggering P0420. This is a valid concern if the misfiring lasted for an extended period.

Anonymous 2010-07-02 08:35

How long had the engine been misfiring before you replaced the coil pack compared to when the P0420 code first appeared? P0420 specifically relates to catalytic converter efficiency. If the misfire persisted for a significant time, it could have already caused internal damage to the converter.

Anonymous 2010-07-02 16:39

I drove approximately 3 miles after replacing the coil pack and used the code reader to clear the fault. The engine has been running well since then. If there was a genuine issue with the catalytic converter, I would expect the check engine light to come back on — but it hasn’t yet. I’ve driven about 100 miles without any issues. Leefish

Anonymous 2010-07-03 08:03

It’s possible that raw fuel remained in the converter and triggered a temporary P0420 code, even after clearing it. However, I've also seen cases where catalytic converter monitors take longer than 100 miles to re-detect failure and log the DTC again. It's worth giving the system more time before concluding there’s a real issue.

Anonymous 2010-07-04 08:35

The light has come back on with the same P0420 code, but the car continues to run smoothly. What does this mean now? Leefish

Anonymous 2010-07-04 10:13

Unfortunately, in such cases, a new high-quality catalytic converter may be required to resolve the issue.

Anonymous 2010-07-05 05:13

I’ll have it checked by a professional mechanic. It’s hard to find reliable diagnostics here — I agree with your assessment that P0420 points toward converter issues. From reading other similar cases, some owners had to replace the catalytic converter or its sensors before the light would go off. I’m not rushing into any decisions and will update once results are in. Leefish

Anonymous 2010-07-09 14:54

I took it to another garage I trust, and they advised me to drive it for a while to see if the issue resolved itself. Since I was feeling confident about switching vehicles, I decided to trade in my Fiesta and bought a Toyota Auris 1.3L with stop-start technology — now I love it! So there you go — someone else’s problem, really. I’m sure that garage made enough profit from me to justify the change. Good luck, Leefish, and thanks for the advice.