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Mike V.
Mike V., Auto Service Technician
Category: Car
Satisfied Customers: 25666
Experience:  25 years of experience, Certified Subaru Technician and Nissan factory trained.
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2000 Kia sportage 2.0 OBD code 171. System lean, repaired ...

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2000 Kia sportage 2.0 OBD code 171. System lean, repaired vacuum leak at upper plenum. Dealer installed 2 new injectors prior and didn''t seal upper plenum. no indication of any other vac leaks. Idles little rough without a/c on but runs beautiful otherwise
Hi there!I am assuming you have cleared the code and it has returned?Did you check for vaccum leaks using carb cleaner or propane?
Customer: replied 9 years ago.
Reply to carguy1's Post: I checked using carb cleaner. cleared code. drove twenty miles,came back on.seems to come on an light throttle cruise.
OK.This code can be a bit tricky.Since you have already done most of the usual suspects,I want to say they need to look at the O2 sensor outputs.The front o2 generally regulates the fuel mixture,however if the rear one thinks it's to rich it will tell the front to lean out the mixture.Sometimes this makes it look like the front O2 is bad but 90% of the time it's the rear reading wrong.Hope this helps!
Mike V. and 3 other Car Specialists are ready to help you
Customer: replied 9 years ago.
I have a snap on scanner. the front 02 output is within range. did not know the rear could affect it like that. do you know what the rear output should be? thank you.
Hang on.I will see if I can get the info for you.

OK.Here is some info for slow responce on the rear O2 sensor.The max voltage you should see from the sensor is 1.0 volt.That is all it is capable of making for voltage.If it stays lower,toward like.3 volts then it will tell the ecm to lean out the mixture.Now,when I was looking up the P0171 code it also said to check the purge control valve.If it is stuck open it will cause a small vaccum leak that you will not find with the carb cleaner.Just a thought,since you have a scanner,you can moniter the pressure sensor and make sure the voltage is changing while the warm fuel from the return line is going back in the tank.

Diagnostic Trouble Code Diagnosis Charts
P0139Rear O2 sensor circuit slow response
Threshold Value ~ Air/fuel compensation factor (atv) (calculated from rear O2 sensor) out of pre-determined high and low parameters Enable Conditions ~ atv: >0.8 seconds or< -0.8 seconds Calculated catalyst temperature: >752°F Time Requirements ~ Continuous MIL Illumination ~ 2 driving cyclesRelated Items
Front and rear O2 sensor connections reversed.
Faulty fuel delivery system.
Leak in intake system.
Leak in exhaust system.
Faulty Mass Air Flow Sensor ground circuit.
Faulty O2 Sensor.
If any misfire, Purge Solenoid Valve, Mass Air Flow Sensor or O2 Sensor heater codes are present, DO ALL REPAIRS associated with those codes before proceeding with this troubleshooting tree.

1IMPORTANT! Record all freeze frame data before disconnecting any connectors or clearing code(s). Check front and rear O2 sensor for connections being reversed. Are O2 sensor connections reversed?YESSwitch O2 sensor connections
NOGo to step 2.
2Perform Fuel Pressure Test (36 to 40 PSI). Perform Maximum Fuel Pressure Test (>50 PSI). Perform Fuel Pressure Hold Test (21 PSI minimum after 5 minutes). Are fuel pressure tests within specifications?YESGo to step 3.
NORepair as necessary.
3Check for any split, disconnected or perforated vacuum hoses. Also, check PCV valve for proper operation and Purge Solenoid Valve (PSV) for proper installation and operation. Are vacuum hoses, PCV and PSV okay?YESGo to step 4.
NOReplace faulty vacuum hose(s), PCV or PSV.
4Connect adapter cable #K99U-2106-G17 to BOB and disconnect C211 from ECM. Connect adapter cable C211 to ECM and vehicle C211 to adapter cable. Start engine and allow to warm up to operating temperature. While monitoring Rear O2 sensor voltage at BOB pin 39, increase engine to between 2000 and 2500 rpm for 5 to 10 seconds and then release throttle back to idle. Rear O2 Sensor voltage should rise during sustained high rpm period and drop significantly when throttle is snapped shut. Repeat a couple of times to verify Rear O2 Sensor operation. Is O2 sensor switching properly?YESGo to step 5.
NOPerform checks in step 5. If no intake leaks are detected, replace O2 sensor.
5With engine still idling, in short spurts, spray aerosol carb cleaner in the following areas while looking for a long rise in voltage at BOB pin 12 (a rise in voltage that is approximately as long as the spray of carb cleaner indicates some of the carb cleaner was drawn into the intake chamber richening the fuel mixture verifying a leak). Allow enough time between areas checked for carb spray to dissipate.
Do not spray carb cleaner on or near coil or plug wires. An arcing wire or coil could start a fire!
Throttle body gasket
Gasket between intake manifold and cylinder head
Gasket between intake manifold halves
Seals between intake manifold and fuel injectors
Are any air leaks indicated?YESRepair as necessary.
NOGo to step 6.
6Check for exhaust leaks by having a helper hold a wadded rag against tailpipe and listen for exhaust leaks between main catalytic converter and cylinder head with vehicle idling. Are any exhaust leaks detected?YESRepair as necessary.
NOGo to step 7.
7Turn ignition off and backprobe Mass Air Flow Sensor at C129-1. Attach voltmeter positive probe to C129-1 and negative probe to chassis GND. Start vehicle, let it idle at operating temperature and measure voltage drop between C129-1 and GND (35mv). Is voltage drop 35 mv of less?YESGo to step 8.
NOLocate source of high resistance and repair as necessary.
8Clear codes and return vehicle to original condition. Verify any repairs by driving vehicle with KIA Data Pro connected to OBD-II connector and monitoring for pending codes (refer to section 3 of the KIA Data Pro Generic OBD-II Program Card reference manual).