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A pulsing CEL can be several causes, either a low idle, poor electrics, misfire..
 

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Discussion Starter · #3 · (Edited)
Pulsating CEL also occurs at driving speed and will stop temporarily while applying brakes and coming to a full stop. I got a 420 code which could be a misfire? But the engine runs as smooth as a sewing machine even at idle. It does however have poor gas mileage and I thought the 420 code might be the O2 sensors. Thanks!
 

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P0420 is a catalytic converter efficiency code, it's not caused by the O2 sensors, although the O2 sensors do play a part in how the converter efficiency is measured - the fact that the pulsing stops when you brake suggests a defective ground somewhere.
 

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Discussion Starter · #5 ·
So the O2 sensors would not cause a 420 code? But its the cat itself? Again thanks for everyone’s patience.
 

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Simplified explanation - the O2 sensors sense the "unburned" oxygen in the exhaust gas - the first O2 sensor, mounted upstream or before the catalytic converter, controls "fuelling", if it sees more than a certain level of unburned oxygen, it will sense it as a "lean mixture" condition and increase the amount of fuel being injected, if it sees less that a certain level, it senses it as a "rich mixture" condition and reduces the amount of fuel - the output from this sensor is a constantly changing voltage between perhaps 0.2~0.8 volts.

The exhaust gas then passes through the catalytic converter whose job is to reduce the level of unburned & partially burned hydrocarbons, a process which also requires oxygen - the second O2 sensor, mounted downstream or after the catalytic converter is there for the sole purpose of monitoring converter efficiency - the output from this sensor is also a constantly varying voltage, but with a much smaller variation than that of the upstream sensor, perhaps 0.4~0.6 volts.

The ECU compares the signals from the two sensors - if the variations from the downstream sensor closely match those of the upstream sensor, it means there is more oxygen in the exhaust gas coming out of the converter than there should be, so the ECU sets the p0420 code.
 

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Discussion Starter · #7 ·
I assume a bad O2 sensor on the upstream would cause a code, what other factors can cause a good sensor to send a code? I understand a misfire can cause problems but the engine is running very well and the idle is smooth. Another problem is the cel is pulsating and the headlights can go off randomly. When I applied the brakes the headlights came back on and the cel quit pulsating momentarily. Obviously a bad ground somewhere, could a bad ground cause the O2 sensor to be reading info wrong. Thanks!!!
 

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A pulsing CEL can be several causes, either a low idle, poor electrics, misfire..
Guessing wont fix nor will throwing parts at it, time to start testing..

Fordem took the time to supply an explanation, it is now up to you.. What skills and equipment/tools do you have..??
 

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I assume a bad O2 sensor on the upstream would cause a code, what other factors can cause a good sensor to send a code?
This may sound like "hair splitting", but that is not my intention - sensors don't send codes, the ECU sets a code based on information received from one or more sensors - I've already explained the circumstances under which the ECU will set a P0420 code, namely, the signal from the downstream O2 sensor matches the signal from the upstream O2 sensor. If either O2 sensor fails, the ECU should detect it and set the relevant codes, which will not be a P0420.

It's important that you recognize that a code pointing to a particular sensor does not mean that sensor is defective, it's an indication that the ECU has detected a problem with the "data" received from that sensor, the sensor could be good, and providing valid data, but the data presented is not what the ECU is expecting at that point in time.

Why are you focusing on O2 sensors when there is nothing pointing towards an O2 sensor issue? Is there something you know that has not been shared with us?

I understand a misfire can cause problems but the engine is running very well and the idle is smooth. Another problem is the cel is pulsating and the headlights can go off randomly. When I applied the brakes the headlights came back on and the cel quit pulsating momentarily. Obviously a bad ground somewhere, could a bad ground cause the O2 sensor to be reading info wrong. Thanks!!!
This vehicle uses four wire O2 sensors, so they have their own, separate grounds, wired directly to the ECU - a defective ground that impacts both the headlights and the check engine light is unlikely to have any effect on the O2 sensor signals.
 

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Discussion Starter · #10 · (Edited)
Thank you, I may have found the problem, a new upstream sensor was installed by the previous owner or owners. The replacement sensor’s plug apparently did not match so they splice the old plug in. I fiddled with the wiring and the next day the engine light disappeared, I scanned and found no codes? Today I’m going to give it a good drive with the scanner and see what happens. I’m also sorry I don’t have your understanding of of these machines, I feel you are talking down to me and I’m not amused 😒 Thank for your difficult to understand explanations , I know you might get tired of answering the same questions for years but no reason to take it out on someone new. Maybe you need to find a different hobby? This is my last post on this subject, I’ll take it somewhere to get fixed or sell it. Keep in mind most here join to get answers and hopefully skip the lengthy research to do the repairs. Thanks again and have a blessed day🙂
 

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The upstream sensor connector has four wires, two for the heater, one for the O2 signal and the last for the signal ground - a poor or no connection on the heater circuit will be reported as a P0135 (B1S1 heater circuit malfunction), a poor or no connection on the O2 signal or ground should show as one of the following P0130 (B1S1 Circuit Malfunction), P0133 (B1S1 Circuit Slow Response) or P0134 (B1S1 No activity detected), and any of these conditions would most likely have affected both fuel consumption & engine performance.

As mentioned previously the O2 sensor wiring is unrelated to the grounds that relate to the headlight and check engine light.
 

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Discussion Starter · #12 ·
Ok, I’m getting a 420 code and a 440. So the O2 sensor are doing their job, if so than the cat is at fault or that’s what I get. I really don’t want to spend $500 on a new cat😞 I know I know test! I just don’t know where to begin. Would taking the downstream sensor out of the exhaust flow using a spark plug non fouler work. We don’t do emissions testing here.
 

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If you have the B1S1 and B1S2 both on that screen shot the CAT is dead..

Post the B1S2 separately..
 

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Sorry to inform you, you CAT is dead... and the engine is running lean..? Or, you may have an exhaust manifold leak.. !

Either way, the CAT is still dead...
 

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Discussion Starter · #17 ·
Thanks for my dead cat 🐱 😁 Would taking the downstream sensor out of the exhaust flow by using a non fouler plug keep the code off?
 

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In short, DOUBTFUL !!
 

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You need to understand what the different PIDs represent, overlaying those that do not correspond negate their viability.

Post the ones you wish analysed separately..

Just looking at the presented jumble the B1S2, is not functioning, it should report between 0.5 to 0.7..with little fluctuation.. This reports the efficiency of the CAT.
The STFT at -0.8 is irrelevant unless we also see the LTFT and know the RPM of record and time since the codes have last been reset.. This reports the efficiency of the fuel burn..
 
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