P0170/P0171 and oxygen sensors

[Pajamarama]

So yesterday my Check Engine light came on in my 97 Accord. Two codes:

P0170 - Fuel Trim Malfunction - Bank 1
P0171 - System Too Lean - Bank 1

I have checked vacuum lines and PCV valve and can't find any leaks. This car doesn't have a MAF sensor to clean or replace. From what I read, this error can also be due to a faulty upstream O2 sensor.

I can order an OE Denso O2 sensor for about 35 bucks, which is cheap enough to try before I give up & take it to the mechanic.

My question is: do O2 sensors need to be replaced in pairs (upstream and downstream) or is it OK for me to just replace the upstream one?

Thanks for any advice and guidance.

 

[Recalculating]

It is OK to change only the upstream O2 sensor.

Ask to loan the O2 sensor socket to remove the old and install the new from the parts store.

Clear the codes.

 

[salv]

Your problem could still be upstream of the O2 sensor, especially if you don't have a fault code for bank 1 sensor 1 O2. Lack of a fault code means that what sensor 1 is reading and what sensor 2 are reading are in agreement. What sensor 1 is reading is an improper fuel and air mixture based on what the PCM calls for. What is most likely happening is that the fuel/air metering is far enough off that the PCM cannot compensate.
-check your entire air intake from the airbox.
-check intake manifold for leaks
-check for fuel pressure
-check fuel pressure regulator
-check for clogs in fuel system
-check injectors for proper function
-check for proper valve adjustment
-check for proper egr function
Lean codes are a royal pain if there is nothing immediately visible. It could result from anything related to fuel delivery, air intake, or exhaust gas recirculation. If there wer codes other than lean codes, that would make life easier.

 

[55Test]

I had a problem on my 98 Civic where I would get random periods of extreme engine hesitation (and massively negative fuel trim). Trouble codes indicated problems with the front oxygen sensor s o I replaced it with a cheapie aftermarket part. I replaced it twice and still randomly got that trouble code. Turns out the rear oxygen sensor was bad enough it was making the front one read funny at times.

 

[LeakySeals]

Better off not replacing the rear/downstream o2, old and fouled = less sensitive for the dreaded P0420 catalyst efficiency CEL. Before replacing the front o2, spray some carb cleaner around the intake manifold and injector ports. If it struggles, replace the gasket(s). Then hunt down vacuum leaks, then finally replace the front o2.

 

[mattd]

If possible, check to see what exactly the fuel trims are. Other data would be helpful also, such as freeze frame and 02 voltages. Scan tools are most helpful here. I would first check for intake leaks, at the gasket where it meets the cyl head. You need to check it cold, because they usually leak when cold and seal up once warm.

 

[mechtech2]

Just get the upstream O2 sensor.

This may also be a fuel pump or filter thing.
Vacuum leaks are #1, so recheck the lines, . connections, and hard to get to stuff, like injector seals.
A manual and visual check is best.
A little propane [unlit!] or carb cleaner can work in short spurts to see an idle RPM rise and tell you there is a a leak. Make sure it is not from being ingested n the normal intake route , thereby fooling you.

 

[vssjim]

using a scan tool if you think the upstream sensor is reading incorrectly look at the downstream. The upstream is the control for the engine the downstream is the check. If upstream is really wrong the downstream will be the oposite. The bottom should be rich as the ECM will add fuel to try to get the upstream to go rich but if it keeps sending a lean signal it adds more fuel and downstream will show really rich. If downstream still shows lean then you are lean as both show the same trend. This old of a car the downstream sensor is only used as checking device some newer cars now adjust fuel trim with A/F sensor upstream and O2 downstream and adjust fuel to keep Cat happy unlike older cars just to check the Cat.

 

[LeakySeals]

Originally Posted By: vssjim
using a scan tool if you think the upstream sensor is reading incorrectly look at the downstream. The upstream is the control for the engine the downstream is the check. If upstream is really wrong the downstream will be the oposite. The bottom should be rich as the ECM will add fuel to try to get the upstream to go rich but if it keeps sending a lean signal it adds more fuel and downstream will show really rich. If downstream still shows lean then you are lean as both show the same trend. This old of a car the downstream sensor is only used as checking device some newer cars now adjust fuel trim with A/F sensor upstream and O2 downstream and adjust fuel to keep Cat happy unlike older cars just to check the Cat.

I know this is not the case with my 02, rear sensor is capped off. Don't understand how this would ever be the case. What would the benefit be setting fuel trim based on post cat emissions? The cat will always skew the results more or less based on conditions and age of the catalyst. The only static accurate reading would be the front o2. Please explain thanks.

 

[Pajamarama]

The two codes and CE light cleared today. What did I do? I filled up with a new tank of gas, that's it.

 

[mattd]

possible last fill had too much ethanol in it, causing a lean condition. did the cel come on when the tank got low?

 

[Pajamarama]

Originally Posted By: mattd
possible last fill had too much ethanol in it, causing a lean condition. did the cel come on when the tank got low?


It came on when the tank was roughly half full/half empty.

 

[LeakySeals]

It could be you adjusted something and the problem resolved it self after a few drive cycles and the gas fill was coincidence. If it turns out to be a gas issue I would consider some techron concentrate plus to clean out the system.