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Code P0421 catalytic converter

Tiny
KIWASABI1
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I never fixed the P0421 either by trying the Liqui Moly catalytic cleaner product, or by replacing the catalytic converter itself. Funny thing is that I passed the emissions test with no issues a couple months back. I recall from this thread that it was determined that the front catalytic converter is bad, and the rear must still be functioning correctly, given our testing and the fact I have no symptoms of a bad catalytic converter (and now I've passed the emissions test).

I've now got a P0135 as well which made me think back to this thread where you guys had me test the upstream and downstream o2 sensors with my ELM327 scan tool along with the OBD2 Car Scanner Android app. How can I tell that the upstream sensor is bad? Should I test from a cold start or with the car fully warmed up when testing the upstream o2 sensor? Just a steady idle or should I rev the engine? From a video I was watching, the voltage should steadily fluctuate between.1 and.9 volts approximately, and if it's flatlined then the sensor is bad.
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Tuesday, April 12th, 2022 AT 12:28 AM
Tiny
STEVE W.
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P0135 would be a heater circuit code for the upstream sensor. It is set when the ECM decides that it's taking too long for the sensor to heat up from the heating element inside it and start switching. It does that by checking the current and sets the code if the heater current continued to be lower than 0.2 ampere or higher than 3.5 ampere for 6 seconds. The sensing part is still working so on the scan tool it will look normal. I don't think that system has the bi-directional ability to turn the O2 heaters on and off to make testing easier. The way it works is simple, the red wire should have battery power with the key on. This same power feeds the crank and a couple other sensors so it should be okay, the brown with white stripe wire is the control wire from the ECM, it grounds to turn the heater on. Same thing happens on the blue with white wire on the downstream sensor.
Testing with a scan tool would be simple, connect a test light to the bed and brown with white and turn the heater circuit on, if the light comes on, that part of the circuit is okay, and you replace the sensor. Without a scan tool you are sort of stuck with just unplugging the sensor and doing a resistance test to see if the heater is still intact. For that you unplug the sensor and use an ohmmeter across the terminals to find if the reading is between 4.5 - 8.0 ohms. If it's lower it's shorted if it's higher it's either open or has a wiring corrosion issue, in either case you replace the sensor. Check that you have battery voltage at the red wire.
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Tuesday, April 12th, 2022 AT 1:27 AM
Tiny
KIWASABI1
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Thanks for that info Steve. I've been getting some conflicting info about potential symptoms from a failed heater circuit on the upstream o2 sensor. Does this cause problems with the air / fuel mixture and with that, rough idle, high idle, low idle, etc? When the car starts up, the idle is lower than it should be, like 800-900 rpms, when normally it would be more like 1200-1400 during a cold start and while in park. Oddly, when the car is fully warmed up and I put it in park, like when I got home tonight, then the idle in park was where it's supposed to be, 1200-1400. Not sure if this is possibly related to the P0135 code or not. Been battling high and low idle issues for years with this car. Actually I also have a P0507 code now since my mechanic replaced the intake manifold, throttle body, and fuel pump, which from what I've been reading, the P0507 is usually indicative of an intake leak. Which could mean the used intake manifold I got is not a good part. I'm throwing lots at you here I know, sorry about that.
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Tuesday, April 12th, 2022 AT 2:20 AM
Tiny
STEVE W.
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The O2 sensor not heating up and operating properly can cause cold start issues like you are seeing. It basically means that the computer isn't going into closed loop as fast as it should and will cause issues. P0507 could be something as simple as a vacuum leak causing high idle.
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Tuesday, April 12th, 2022 AT 3:31 AM
Tiny
KIWASABI1
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After I replaced my upstream o2 sensor that got rid of both the catalytic converter code and the o2 sensor code for months. But now I got 2 new P0421 codes and 1 new P0442. I had replaced the fuel pressure regulator hoping that would clear the P0442, but alas, no. I have freeze frame data for one of the P0421 codes:

FUELSYS1 CL
FUELSYS2 N/A
Load Percentage 33.7
ETC 201
SHORTFT1 Minus 14.8
LONGFT1 0.8
RPM 2195
Speed 52 MPH

What sticks out there is SHORTFT1 being minus 14.8. Seems high. Anyway I've had that LiquiMoly Catalytic Converter Cleaner stuff sitting around for months now. Should I try it? I just got a new intake manifold and I'm a little worried about putting that stuff into my air intake (it gets sprayed into the throttle body). If you look above, we found out my car is a California car originally and has 2 catalytic converters. I don't have any black smoke or anything but I do have some engine hesitation when the car is cold and just starting to accelerate. I also still have a rough / hunting idle that fluctuates between 700 to 1000 RPMs depending on whether I have A/C on or not. And lots of vibration as a result of that as well. I replaced the motor and transmission mounts last year so it's not caused by that. I think it's all related. I'm planning on replacing the downstream o2 sensor and oil pressure sensors as well.

Thanks,

Adam
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Thursday, September 29th, 2022 AT 5:12 PM
Tiny
STEVE W.
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The 421 code is saying that the converter isn't heating up fast enough, that could be from the extra fuel and the rough cold start. The 442 is an EVAP small leak code, I would start by checking the purge valve at the engine end. If it is leaking a small amount it could be dumping fuel vapor into the intake while it sets and then you get the rough cold start. I would also check on the bank one downstream sensor. If it is damaged or has an issue you can get a 421 until it heats up.
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Thursday, September 29th, 2022 AT 11:24 PM
Tiny
KIWASABI1
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Hi Steve,

I wanted to give an update on this thread. I ended up replacing both of the O2 sensors which cleared all my O2 sensor codes. My fuel filler neck was rusted out and started causing a fuel leak while gassing up, so I replaced that with an OEM one and that fixed the P0442 code as well as the leak. I haven't wanted to replace the catalytic converters as they do seem to be working. So, I bought some Dura Lube Severe Catalytic and Exhaust Treatment Cleaner which I'm going to try. If that doesn't work, then I will finally try the Liqui Moly 8931 Catalytic-System Cleaner. I've been reluctant to spray it into the throttle body as I was warned if it pools up that it can cause damage. But I bought the stuff 3 years ago as well as a Hudson sprayer and just haven't committed to trying it yet. Anyway, hopefully one of these products will fix my P0421 since that seems to be a much less severe code than P0420.

Thanks,

Adam
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Wednesday, August 23rd, 2023 AT 1:25 PM
Tiny
STEVE W.
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Thanks for the update.
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Wednesday, August 23rd, 2023 AT 2:11 PM

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