Hi all --

Same old Fiesta from 2012, still going. I've had a new cat installed last year (by a supposedly proper garage... i.e. costly), and yet last week the P0420 code popped up. The car drives just fine (for a 15 y.o. one anyway). MOT is in a couple of months, so I need to fix this code. Am I correct that chances are it's the upstream lambda sensor? I've seen this code 15 years ago on another car, and replacing the upstream sensor as DIY fixed it back then (that said, that car had 250,000 miles on it, not 120,000 like this fiesta).

Is there anything I should check before going ahead with the sensor? I see messages here that mention the sensor heater, the heater/sensor wiring and the respective fuse. Which reading from the sensor would I check to confirm it's likely gone -- is it the voltage? I believe I did this 15 y.o ago, but I no longer remember what exactly, should have taken notes :(

Also, the original FORD sensor is unjustifiably costly (I see online prices above 120 quid). Would I be OK with a generic one from a reputable supplier, and if yes, any recommendations? A Bosch one I found seems to come w/o leads, so I guess I could solder the old header to the new sensor. That said, I've never done soldering of components exposed to harsh elements, so I am not sure if generic shrink tubing would be OK (and won't raise red flags with the MOT people).

Cheers!

What brand of catalytic converter was installed?

If it was an aftermarket one, that will most likely be the problem. Aftermarket catalytic converters are often made of thinner steel and contain considerably less of the valuable metals compared to the original catalytic converters.

Installing an aftermarket catalytic converter on a 1.0 EcoBoost is usually only a temporary fix. I have seen many times before that an aftermarket catalytic converter only lasted for a short period on a 1.0 EcoBoost.


An original catalytic converter approximately costs GBP 750.- excluding installation. If the installed one was less expensive it is most likely not an original one.

Thank you, JW. I am almost certain an original one was installed for this job, need to check the records. It was not a dodgy little shop.

If it is indeed an original catalytic converter it would be a start to post the full DTC code including the suffix.


The P0420 DTC code itself only indicates:

"Catalyst System Efficiency Below Threshold (Bank 1)"

This is quite a generic code. The suffix can help to narrow it down.

JW, OK. I will see what other data my old reader provides. From what I could see "Catalyst System Efficiency Below Threshold (Bank 1)" was all it showed.

OK, so the cat was not a FORD part, but was not a shady part, either (the cost of the part was around 500 quid). There is a 2-year warranty on the cat, and, as it turns out, it was replaced 22 months ago (even though it seemed very recent in my memory). Maybe should've asked for a FORD cat, my bad.

I will look at the voltages on the O2 sensors in real time to learn more about this. If it's the cat, I might argue for a warranty repair with the shop (if only on the cat, not the labour)

For what it's worth, I took it on a 15-min drive on a local highway. O2S12 (I presume it's the downstream sensor) hovered around 0.5-0.6V at 2000 rpm with normal highway driving. When I ease off the throttle it goes down to 0-0.1. The O2S11 (must be the upstream sensor) displayed data in mAmps, not in volts (?), and that fluctuated around zero (up to 2mA, but usually much less), not sure if there is any pattern in this data, to me this looks like there is not much current through. The check engine light did not come up during this short drive.

55 minutes ago, runcyclexcski said:

For what it's worth, I took it on a 15-min drive on a local highway. O2S12 (I presume it's the downstream sensor) hovered around 0.5-0.6V at 2000 rpm with normal highway driving. When I ease off the throttle it goes down to 0-0.1. The O2S11 (must be the upstream sensor) displayed data in mAmps, not in volts (?), and that fluctuated around zero (up to 2mA, but usually much less), not sure if there is any pattern in this data, to me this looks like there is not much current through. The check engine light did not come up during this short drive.

The Front Sensor is a Wideband one. They are fed a Current and don't produce a Voltage like the old style ones and that Current should be close to zero during steady driving and not deviate by much or for long when accellerating/decellerating.so your one looks to be performing exactly as intended.

Tizer -- OK, many thanks. Do you happen to know what condition of the downstream sensor would trigger the error? Is it a cumulative persistence above a threshold for a certain period of time... per distance driven? I mostly do city driving with this vehicle and only go on the highway once or twice a month.

The PCM will try to monitor the Catalyst once or twice per driving cycle if the driving and vehicle conditions are correct for monitoring, how it works out if there is a problem is beyond me though.

If there is a problem with one of the O2 Sensors I would expect a different Fault Code and there may not be a P0420 Code because the Catalyst Monitor may not be allowed to proceed in those circumstances.

A little information that may help. I have a 2017 Fiesta 1.0 Ecoboost 140 and had the P0420 (cat below efficiency) first popped up in November. I cleared it with Forscan but the message started to come back more and more often. It was getting towards the MoT so I took it to the main Ford dealer. They checked my exhaust system for leaks and told me there was a "PCM update" available for the car and did I want to pay for that to be done (as it's much cheaper than just fitting a new cat). I agreed and it was done. It's been almost three months now and touchwood the fault code has not yet returned. From speaking to the chap at the service desk the update "recalibrates" the system so it's less fussy about the cat. My car have since had an MoT at a small local garage and they confirmed the emissions are absolutely fine and well within limits.

Even with the P0420 DTC code active most 1.0 EcoBoost engines do still comply with the emission standards.

If the problem turns out to be caused by the decreased efficiency of the catalytic converter and it is not covered by the warranty, I would try an O2 sensor spacer first before doing anything else.

An O2 sensor spacer is basically just an adapter that is placed between the catalytic converter and the O2 sensor. It either has a smaller hole or any other form of restriction (like being filled with steel wool). These can be quite effective.

Yeah, I've read about those spacers. Not sure how legal they are?