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Ford Fiesta 2004 1.4i Zetec P0420 Code: Catalyst Efficiency Below Threshold Diagnosis and Solutions

Model: Ford Fiesta Fault Code: P0420 Posted: 2011-04-06 12:23

Hi everyone, I'm posting about a recurring issue I've been experiencing with my Ford Fiesta 2004 1.4i Zetec engine (current mileage: 45,000 miles). The vehicle is showing a P0420 fault code, which stands for 'Catalyst System Efficiency Below Threshold (Bank 1)'. I've conducted an OBD-II diagnostic using a scanner connected to my laptop. After warming up the engine by driving it for 20 minutes, I recorded the oxygen sensor voltage outputs. The front O2 sensor shows consistent fluctuations between 0.1V and 0.9V and responds almost instantly to throttle inputs. The rear (downstream) O2 sensor, however, exhibits much flatter readings with minimal variation. I've attached two graphs: one showing idle at 750 RPM and another during acceleration from 1500 to 2500 RPM. In both cases, the blue line represents the front O2 sensor and the yellow line the rear O2 sensor. At idle, the time between a high voltage reading (~0.9V) on the front sensor and its subsequent drop is approximately 3–4 seconds. I'm seeking expert input to determine whether this behavior indicates a problem with the oxygen sensors or points directly to a failing catalytic converter. If anyone has experience with this specific model (Ford Fiesta 2004 1.4i Zetec) and can offer insight into interpreting O2 sensor waveforms in relation to P0420 codes, I'd greatly appreciate it. Additional details: - Front O2 sensor: responsive, normal voltage range - Rear O2 sensor: flat response, not oscillating as expected - Vehicle is out of manufacturer warranty (6.5 years old) Graphs available at: http://www.screencast.com/users/JonScot ... 72ce3daf28 http://www.screencast.com/users/JonScot ... 38fae2db3e Thanks in advance for your help! P.S. I'm using Easy OBD-II software to capture the sensor data.

Related fault codes
P0420
Comments (9)
Anonymous 2011-04-06 13:11

The catalytic converter is typically covered under an 8-year or 80,000-mile warranty, depending on the region and vehicle model. However, this may not apply to your situation due to age and usage.

Anonymous 2011-04-06 14:44

Thanks for the reply! I've already contacted Ford as the car was purchased new. Their official policy states a maximum warranty of 5 years or 100,000 miles (whichever comes first), which must be purchased separately on top of the standard 3-year coverage. Since my vehicle is now 6.5 years old, it's well beyond warranty. More details are available here: http://www.ford.co.uk/Buyingandprotecti ... edWarranty. Unfortunately, this means I'll have to cover any repair costs regardless of whether the issue turns out to be a sensor or converter failure. Appreciate your input.

Anonymous 2011-04-07 07:39

The oxygen sensors are likely functioning correctly — the P0420 code is most commonly caused by a failing catalytic converter, not faulty O2 sensors.

Anonymous 2011-04-07 12:08

Hi Mohawkmtrs — could you please explain how you arrived at that conclusion? I'd like to understand the reasoning behind your assessment.

Anonymous 2011-04-07 19:42

It's a common misconception that faulty O2 sensors cause P0420 codes. When the downstream (post-cat) O2 sensor begins oscillating rapidly and starts to mirror the upstream O2 sensor’s pattern, it signals that the catalytic converter is no longer performing its function properly. This behavior often occurs when the converter has degraded or failed. I've seen cases where fixing upstream issues like misfires or fuel control problems resolved P0420 codes — but never due to a faulty O2 sensor.

Anonymous 2011-04-08 07:33

For clarity: post-cat (downstream) O2 sensors should exhibit long, slow wave patterns. If they begin to mimic the upstream O2 sensor’s fast oscillations, it's a clear sign that the catalytic converter is not working effectively. Since you have access to graphing software, I recommend re-running diagnostics after installing a new catalytic converter and comparing the waveform patterns — the differences will be very noticeable. Also, consider purchasing a high-quality catalytic converter, preferably from an authorized dealer (cheaper aftermarket units are often of lower quality).

Anonymous 2011-04-21 06:26

I have a 2003 Ford Fiesta 1.4 Zetec with 36,000 miles and the same P0420 code. The engine has a rich fuel smell. I recently found a used post-cat O2 sensor on eBay for just £1 (a bargain!). I plan to try it first before replacing the catalytic converter — hoping it will resolve the issue without major cost. What software are you using? I’ve only tried Easy OBD-II, which came with my scanner. Hope it's not the converter — I’d rather avoid spending money on a full replacement until it fails an MOT.

Anonymous 2011-04-23 05:50

I've installed the £1 post-cat sensor and now see much lower voltage switching compared to the pre-cat sensor, which suggests it's working. The old sensor was covered in black dust — likely indicating a long-term failure or poor airflow.

Anonymous 2011-06-01 06:00

The P0420 error has disappeared after installing the used post-cat sensor — looks like the issue is resolved for just £0.01! Thanks to eBay and everyone who shared their insights.