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Open/Closed Loop Operation

2002 Toyota Camry LE 2.4 A/T 85k miles. all maintenance up to date.

I got a CEL P0420 last night, car ran fine for 85 miles back to house. Hook scan tool up to look at sensor outputs, and found that the A/F ratio sensor is N/A. And that the car is never going into open loop. Coolant temp stays around 185 at 55 mph and will only hit 200 if I hold it at around 2500 RPM in neutral while stopped. Even on the highway it stays in closed loop. I know about the P0420 catalyst code every Camry I have ever had eventually gets it. Last 2 Camry’s needed downstream O2 sensors 1 Camry got totaled before i resolved the P0420. But could it be caused by staying in closed loop for extended periods of time? Should I replace the thermostat to try to get the engine temp up? Any ideas?

If I understand you correctly, the system is staying in “closed loop” mode when hot. That’s correct. The only time the system should be in “open loop” mode is when cold. “closed loop” means the injectors are reacting to the AF sensor, “open loop” means the sensor signal is being ignored.

I’m pretty certain that you have the same engine I do. Believeing that, I’ve attached some diagnostics procedures that should help. The first link describes the procedure for the SFI system, the second provides an index of and links to other diagnostic procedures some of which may be of help. I hope this helps.

tsm is correct. Sensors don’t work well ( or sometimes at all) until they’ve reached a minimum operating temperature. Until that happens, the engine runs on default settings without checking to see if those settings are producing the desired results. Once the sensor warms up, the engine can use the sensor to check its output and adjust accordingly, a process known as a closed loop.

Thank you both, I guess I had open and closed backwards, and panic-ed too soon. I have cleared the code and will see if it returns.TSM I can not get those links to work. In the interm I will try to get better scanner software so I can compare up and down stream sensors in real time since the P0420 will probably return at some point. Thanks again.


Thanks fot the feedback. I’ll see what I can do with the links. I’m not much of a computer geek, however, so I can’t promise anything.

See if the below is better.

No luck on the link. I will see if I can get a decent service manual, thanks for trying


Pvt, if you have an Android tablet or phone, take a look at Torque. $5 app, and a $25 bluetooth OBD2 scanner from Amazon and you have a very good scan tool for much less than a normal scan tool.

One more try…
If this opens, go to the repair manuals, diagnostics submenu.

tsm: lets you shorten urls into more standardized forms so that bad forum software won’t get confused by them. Try it next time.

UMmmmmmmm…let me get my tranlation dictionary…
I’ll try as soon as I can find out what all that means.

hehe. Translation: Paste a URL into the box at and it will spit out a shortened version that won’t confuse discussion forums, which you can then copy and paste here.

There is a service bulletin on that car for a software update and a redesigned catalyst for a P0420 fault. Unless it becomes a nuisance I would just clear the fault when it comes up.

A thermostat is worth a try after 11 years.
They’re not expensive.
I recommend OEM.

Here’s an idea . . . if the code reappears why don’t you have the software updated but hold off on the catalytic converter?

Then if it consistently comes back, keep the updated part in the back of your mind.

Another thing to consider is that there are several possible causes for a P0420. A catalytic converter is often the very last step after ruling out everything else.

Have there been any misfires? Severe misfires could damage a catalytic converter.
Excessive oil consumption?

No mis-fires, no excessive oil consumption. The last Camry I had with this code, a new downstream O2 sensor cleared it up. I have reset the CEL, if the code reappears I will look into the recalib. Thank you all for the suggestions.