2005 Malibu Maxx with P0171 and P0174 runaround



I have been tormented by this car, a 2005 Chevrolet Malibu Maxx with 200,000 miles and it’s P0171 and P0174 problems for over 6 months. In this time it has been in the shop multiple times and it still throws the errors. I have seen code P0507 once… But only once… These are the only codes it throws.

Please note that other than a slightly rough idle at 7-800 rpm and and an annoying jingle from the muffler at 1500 rpm the car runs perfectly.
At least it does until it throws the “Too lean” error codes and dies at the side of the road. Then once the codes are cleared it works perfectly until the next time. Which can be as little as 2 miles in stop and go traffic or 2 hours on the freeway.

This all started when idiot me put bad gas in the car. I eventually worked it through, changed out the plugs and wire set and ran a few cans of Sea Foam and fuel dryer through the car in several tanks of gas. However since then I have had this reoccurring problem of the error codes.
Chevrolet’s notorious 3 bings and “ENG PWR REDUCE” and basically your car turns into a paperweight until the codes are cleared.

Since then I have replaced the throttle body, Mass Air Flow sensor, confirmed the air filter is very clean AND purchased my own wireless OBD II diagnostic tool so I don’t get stuck at the side of the road any more… But the problem is still happening.

My mechanic has examined the engine and says there are no vacuum leaks in the engine and it appears to be sound. He says that he cannot find anything obviously wrong with it to fix and to drive it until it throws an error we CAN fix…

Thus I am driving it around running scans using the OBD II scan tool… Which, thank the lord, is reporting something! WHAT DO THEY MEAN!!! HELP!!

I have a guess that they might indicate a stuck open EGR valve? Maby? But what do I know… I studied computers not cars…

thanks for any help or suggestions!!
Sean in Cincinnati

All test not listed passed or were incomplete.
I have checked all fuses and none are blown.

MID:$21 TID:$01
Rich to Lean sensor threshold voltage(constant) - Catalyst Monitor Bank 1
Max: 41,243mHz Min: 20,512mHz
Test result value: 288mHz

MID:$21 TID:$03
Low sensor Voltage for switch time calculation - Catalyst Monitor Bank 1
Max: 41,245mHz Min: 4,128mHz
Test result value: 800mHz

MID:$31 TID:$5f

  • EGR Monitor Bank 1
    Max: 40,973 Min: 26,816
    Test result value: 14,080

MID:$41 TID:$1b

  • O2 Heater Monitor Bank 1 - Sensor 1
    Max: 41,113 Min: 20,528
    Test result value: 6,976

MID:$50 TID:$03
Low sensor Voltage for switch time calculation - O2 Heater Monitor Bank 4 - Sensor 4
Max: 45,072 Min: 6,162
Test result value: 65,520

MID:$91 TID:$5f

Max: 40,968 Min: 26,816
Test result value: 14,080

MID:$a1 TID:$01
Rich to Lean sensor threshold voltage(constant) - Misfire Monitor General Data
Max: 41,243 Min: 20,512
Test result value: 416

MID:$a1 TID:$03
Low sensor Voltage for switch time calculation - Misfire Monitor General Data
Max: 41,245 Min: 20,512
Test result value: 928

MID:$a1 TID:$04
High sensor Voltage for switch time calculation - Misfire Monitor General Data
Max: 41,246 Min: 4,128
Test result value: 1,184

MID:$a1 TID:$0a
Sensor period(calculated) - Misfire Monitor General Data
Max: 41,252 Min: 4,128
Test result value: 2,720

MID:$b1 TID:$08
Maximum sensor Voltage for test cycle(calculated) -
Max: -48,572mV/s Min: 8,256mV/s
Test result value: 4,448mV/s

MID:$c1 TID:$19

Max: 41,267 Min: 20,512
Test result value: 6,592

MID:$c1 TID:$1a

Max: 41,113 Min: 20,528
Test result value: 6,848

MID:$c1 TID:$1b

Max: 41,113 Min: 20,528
Test result value: 7,104

MID:$d1 TID:$32

Max: 1 Min: 20,561
Test result value: 13,008

MID:$e1 TID:$33
Manufacturer defined MID -
Max: 41,113 Min: 20,528
Test result value: 13,280


When I see that many monitor failures from the computer, I raise an eyebrow towards the computer itself.



Some of the tests seem to indicate the heaters for the O2 sensors may not be working. I suggest you check the fuse that provides them power and make sure power is getting to the heaters. Please provide the error codes you are seeing also.


I have checked all the fuses and none are blown…
The car only throws the 2 main errors regularly. It has once thrown a P0507 and that was right after the oxygen sensors were changed.


The “bad gas” episodes are always suspect (often blamed when no one can find another cause for a problem, but if you did have a known fuel system problem it certainly could go back to that. Lean codes can come from too much air - or not enough fuel. Either way.

Has the fuel pressure been checked with a fuel pressure gauge? Is it within specs? Was the fuel filter changed? You might want to worry about the fuel injectors. There are off-car services that will clean and flow test them. (The car will be disabled for a while as most of these are mail order). There’s no way to after the injectors without spending some $$. But in any case, I would be worrying about fuel delivery.


If the fuel pressure checks out ok, I’d be suspicious of the O2 sensors. One or more of them may be failing. Are they original to the car? The EGR code looks suspicious too, but it may be a red herring. If I got that code on my Corolla to rule it out as a cause I’d verify that the EGR was moving like it should when vacuum was applied, and applying vacuum to the EGR would stall the engine at idle. If the fuel pressure and EGR checked out, I’d run a compression test, and check all the vacuum operate devices for diaphragm leaks with a hand held vacuum pump. If the same codes kept appearing, and this only happens when the engine is warm, I’d probably bit the bullet and replace the O2 sensors. The bad gas or the gas treatment chemicals later may have damaged them. And @200K miles, they should probably be replaced soon as preventative maintenance anyway.


Thanks for the advice… The O2 sensors have already been replaced. I will clean the EGR today and see what happens.


P0171 and P0174 should not make the car stall, unless it’s a massive vacuum leak, then simply resetting the code would not make it run again.
I suspect the computer itself, or the power & ground lines to it.


A faulty alternator can play hob with the ECM (computer). A bad diode or bad spot on the commutator can cause the power output to drop out intermittently, which confuses the ECM to no end. The ECM needs a nice steady power signal to work correctly. It’s difficult to diagnose the quality of the power signal output from the alternator without the aid of a laboratory o’scope, but if you had temporary access to a known good alternator, might be worth a go to give the alternator a swap, see how that affects the symptom.


Circuitsmith - The car did not stall the error ENG PWR REDUCED would come up on the radio/system display and the car computer would disconnect the electronic gas pedal.

George – I will have Autozone do a charging test and see what can be seen. I do not have another alternator… but thanks for the suggestion

Today I pulled and attempted to clean the EGR. It is massively pitted, corroded, rusted and simply will not clean. So I ordered a new one. On reinstallation so I still have a car the car did run better and did not throw any errors in 40 miles… This is an improvement. I will attach the latest failures from that drive… I am getting infinite voltage from the 4th o2 sensor heater… Sounds like there is a short possibly? In addition, the EVAP tests and the Catalyst Monitor Bank tests are not completing.

thanks for all the help


MID:$11 TID:$5f

Max: 40,966 Min: 26,816
Test result value: 14,080

MID:$50 TID:$03
Low sensor Voltage for switch time calculation - O2 Heater Monitor Bank 4 - Sensor 4
Max: 45,078 Min: 22,546
Test result value: 65,520

MID:$c1 TID:$19

Max: 41,113 Min: 20,528
Test result value: 6,592

MID:$d0 TID:$03
Low sensor Voltage for switch time calculation -
Max: 45,075 Min: 6,162
Test result value: 65,520

MID:$d1 TID:$32

Max: 1 Min: 20,561
Test result value: 13,008


I’m certainly no expert but a lean condition indicates lack of fuel if the error codes are correct. I’ve hooked up a fuel pressure tester and taped it to the windshield to be able to read the pressure when the car falters. If you’ve got a pump problem, screen problem, filter problem, etc. if in fact it was from bad gas, the pressure tester should show a lack of pressure before it stalls.


Thanks for the help Bing - The car itself does not stall or act up at all… The cars computer throws an error code, it disconnects the gas pedal and begins to idle roughly. The car never falters. If I then use my code reader to reset the codes (without turning off the car or doing anything else) the car runs correctly again.


Today while driving the car is acting differently (the only change was a cleaning of the EGR valve)

Current Fault Log

P0507: Idle Control System RPM Higher Than Expected
Pending Fault Log

P0171: System too Lean (Bank 1)
P0174: System too Lean (Bank 2)

The P0507 error code keeps popping up (note that the car does not appear to be idling too fast, must be some other reason triggering that code)

One note, in all my mode $06 error test I have never seen the EVAP tests come back with any values. Is this normal for GM or is there a way for me to test this. I do know that just before this all happened the canister purge valve broke and was replaced. The hose to it appears fine…


I did not see a gas filter in the list. Hopefully it is not in the tank.


The gas filter is part of the gas pump and is… in the tank…


Are you able to place the ECM in open loop test mode temporarily? In that mode the ECM ignores the O2 sensors and just control’s the engine’s air fuel mixture like a normal 1970’s pre-O2 sensor engine. I can do that on my Corolla with a simple test jumper. It’s a very simple thing to do on the Corolla. If it was simple likewise for your car, might be worth it from a diagnostic viewpoint to see how the car runs in open loop mode. If it runs ok, and normally runs ok even when not in open loop as long as the error code remains off, that would tend to say the problem isn’t the air/fuel mixture, but the measurement of that parameter; i.e. something wrong with an oxygen sensor or associated circuitry. You did mentions something about the O2 heater circuit. If that isn’t working, the O2 sensor will take a long time to start working, and even when it starts working, it might not produce accurate measurements.


Really good point George. I will look into it!


Has the PCV valve been checked? How about cleaning the IAC? Vacuum test shows OK?

A P0507 DTC trouble code may be caused by one or more of the following: A vacuum leak Leaking air intake after the throttle body EGR valve leaking vacuum A faulty positive crankcase ventilation (PCV) valve Damaged/failed/dirty throttle body Failed EVAP system Failed IAC (idle air controller) or faulty IAC circuit Possible Solutions This DTC is more of an informational code, so if there are any other codes set, diagnose them first. If there are no other codes, inspect the intake air system for air or vacuum leaks and damage. If there are no symptoms other than the DTC itself, just clear the code and see whether it returns. If you have an advanced scan tool that can interact with the car, command the idle higher and lower to see if the engine responds appropriately. Also check the PCV valve to see if it’s blocked and needs to be replaced. Check the IAC (idle air controller) if equipped, verify it’s operation.


Thanks for the help knfenimore. I am now seeing the P0507 code even when driving down the freeway and it is throwing it almost constantly when the engine is warmed up but NOT when it is cold. Tomorrow the EGR get here and I will install that and a new PCV valve and hose.

As for a IAC, I cannot find a controller but there is a motor replacement available. Strange. However since the PCV and EGR show up in every scenario as a possibility I will replace those and see if anything changes first…

thank you all for helping!