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Hey all, I've been fighting this P0420 cold start issue for a long time on our 2000 civic.
Late 2019 I replaced the exhaust manifold & upstream O2 sensor. (pictures of parts attached)
The car has cold start problems. Plugs look good (other than they were covered in gas this morning after it wouldn't start)
I checked the upstream O2 sensor & it had 13 ohms, sounds like 10-20ohms is within specs.
Hooked up my ODB sensor & drove it a bit. Upstream O2 seems to be within spec but the downstream appears not to be. It's supposed to be around 0.5 volts & pretty consistent but mine fluctuates.
I checked the temp before & after my cat. It was hard to get a good reading on the inlet side but it appeared to be close to the outlet side of roughly 350 degrees.
Any idea what my problem is? I'm leaning towards the cat being bad.
Re: Need help diagnosing Catalytic Converter P0420
Is that the lambda reading for the O2 on those graphs? 0.7 lambda would be like 10:1 AF ratio. That's really rich, and that's the highest reading on the graph.
Edit: After a quick Google search 0.8-0.9V is normal, 0.5V and less is running lean. Assuming you were lifting off the accelerator where the graph dips that doesn't look like a problem.
Re: Need help diagnosing Catalytic Converter P0420
Originally Posted by BrotatoChip
Is that the lambda reading for the O2 on those graphs? 0.7 lambda would be like 10:1 AF ratio. That's really rich, and that's the highest reading on the graph.
No, that should be volts. Upstream should be 0-1V & downstream should be relatively constant 0.5V from my understanding.
Re: Need help diagnosing Catalytic Converter P0420
Originally Posted by danmurphymn
No, that should be volts. Upstream should be 0-1V & downstream should be relatively constant 0.5V from my understanding.
Had to edit my previous post. Forgot these don't have wideband O2 sensors.
I would be amazed if this has anything to do with the cat, it shouldn't affect starting only when cold. IAT, ECT, and MAP would all affect cold starts.
Re: Need help diagnosing Catalytic Converter P0420
Originally Posted by BrotatoChip
Had to edit my previous post. Forgot these don't have wideband O2 sensors.
I would be amazed if this has anything to do with the cat, it shouldn't affect starting only when cold. IAT, ECT, and MAP would all affect cold starts.
So they may be two separate issues? That sucks... haha
I think when it's cold it throws too much fuel. It won't spark enough to get it started. I've replaced plugs & wires originally. What are IAT, ECT & MAP & how would I best diagnose them to see if they're my problem? I'm sick of pushing my daughters car inside when it's cold & I don't want to lose my parking in the heated shop
Re: Need help diagnosing Catalytic Converter P0420
Okay, I checked the intake temp & it's right about the temp of my shop so it doesnt appear to be the intake air temp.
MAP Sensor has the correct voltage & seems good. The only thing I couldnt seem to check was if the voltage changed by creating a vacuum while on the voltmeter. I probed it as per the instructions here. https://troubleshootmyvehicle.com/ho...e-map-sensor-3
I tried to create vacuum through some of the intake vacuum hoses but the voltage didn't seem to change.
ECT, I replaced the radiator not too long ago so my assumption is that th ECT is good and this problem has been persistent since before I changed the radiator.
