I have a 2010 Rx350 that throws the codes 0171 & 0174 intermittently. The check engine light will come on along with the traction control light. Weird thing is the check engine and traction control light will periodically go off by itself without me clearing the codes. I took it to the dealership explained the issue and left it for 8 days. They went through all the testing that is required for those codes and found absolutely nothing. They replaced all gaskets related to the possible problem, did a smoke test and plenty of other diagnostic tests, they basically got to the point to where they were ordering parts and replacing then taking them off and returning them because nothing was fixing the issue. So I finally just picked it up because I was going on vacation and didn’t want it to stay there for two more weeks. Has anyone had this problem that they found a solution for?

 

You are very lucky as they are accepting a return after installing part(s).

Lexus dealerships have trained mechanics and they have a regional expert who can be looped in to consult. I would say talk to the service manager.

If you are diagnosing by yourself, try reading live data. Some savvy member may be able to give you guidance.

Salim

 

Just a guess, but maybe an air/fuel ratio sensor is bad. I believe that bad air/fuel ratio sensors can trigger the two codes as well, however it is odd that both banks are bad at the same time. I would also check the wiring harness ground connections for any bad connections.

Personally I would check the intake manifold seals if nothing else solve the problem since a bad intake seal can allow un-metered air to enter the manifold pass the sensors and mess up the air/fuel ratio the ECU is measuring which can trigger the warning for both banks like in your case.

 

On a non-Lexus vehicle we own, P0171 and P0174 codes were resolved by replacing both VVT (Variable Valve Timing) solenoid O-ring seals.

Who is your dealership?

 

I’ve been taking it to Johnson Lexus of Durham.

 

The gaskets I know they replaced was the Plenum manifold and Throttle body, plus they replaced an oxygen sensor and they inspected and cleaned the connectors. I’m not sure if they inspected all other air/fuel sensors or connections though related to the issue.

 

Well, i am at a lost. The codes are generally cleared by replacing those items.

My next best guess is may be a bad ground connection(lose or broken wire) on the chassis.

 

Very probable. Specially if there is not visible ground tap in between the engine and fire wall.

Salim

 

Your long term fuel trim is pretty high, the link below might be helpful for you.

https://www.clublexus.com/forums/rx-...4-codes-3.html

 

Yeah I read his write up and solution, but the part that fixed his issue (Air intake control Valve) doesn’t seem to exist on the 2010 Lexus Rx 350. I guess Lexus changed the intake when they moved on to gen 3.

I appreciate the help!

 

Assuming the dealership ran the smoke test correctly, and ruled out a leak in one of the several vacuum hoses, have they looked at the MAF? Is there a CAI installed by any chance or a nonstandard air intake setup? The MAF is a PITA to change out because the windshield cowl has to come off, just like when replacing the intake manifold, so curious if they switched out or tested the MAF while the cowl was off.

 

They did a smoke test, not sure how well, but it was done. There’s no mods done to it like CAI or anything like that, everything is stock Toyota/Lexus parts. The MAF was one of the parts that they ordered, replaced and then sent it back because it didn’t fix the problem.

 

So after months of dealing with these codes popping up intermittently and the dealership not knowing what the problem is or even where to look I decided to take it to a hole in the wall mechanic shop, like every mechanic is working on cars while a cigarette is hanging from his mouth mechanic shop.

Dropped the car off and took an Uber home, 30 min later, got a call that they found out what the issue was and to pick up the part on the way back and they can fix it today.

When the dealership replaced the valve cover gasket they didn’t seat the gasket properly and that was the reason for the small air leak.

However I reached out to the Lexus dealership and explained what I found and they reimbursed me for the charges to replace the gasket.

 

Glad to hear that Johnson Lexus reimbursed you for their lousy work. Long-time-in-business, very knowledgeable indie shops are so much better than any dealership. The only reason I ever darken the door of a dealership is for warranty work or recalls.

 

And the winner is CarGuy post #3.

Glad it got resolved and dealership owed up to their mistake.

Salim