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Greg, import technician
Category: Hyundai
Satisfied Customers: 6759
Experience:  27 year and 16 year as shop owner, import technician
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2003 Elantra: P0302..both O2 sensors..Electrical connections

Resolved Question:

I am reporting P0302 and P0136(twice) codes on my 2003 Elantra. I replaced both O2 sensors and am still reporting the codes. Electrical connections look clean and no cord damage is visible. Car runs rough, like it has a fuel supply problem or cylinder not fireing correctly. When check engine light first came on it just reported the codes but the car seemed to run fine, but it got worse and worse, like a component failing slowly
Submitted: 5 years ago.
Category: Hyundai
Expert:  Greg replied 5 years ago.
Hi welcome to will answer your question the best I can,

P0136.This code is due to the fact you have been running this engine to long with the misfire P0302.

This will have to be sorted out before going for the P0136 code A misfiring engine will let rare fuel get to the Cat and if this happens to long then the cat sure will get damaged .

So P0302
Component Location

General Description
The Misfire monitor diagnostic is based on crankshaft rotation velocity variation. The PCM determines crankshaft rotational velocity using the crankshaft position sensor and camshaft position sensor. When a cylinder misfires the crankshaft slows down momentarily. By monitoring the crankshaft and camshaft position sensor signals, the PCM can calculate when a misfire occurs. For a non-catalyst damaging misfire, the diagnostic will be required to report a misfire present within 1000-3200 engine revolutions. For catalyst damaging misfire, the diagnostic will respond to monitor 200 engine revolutions. Rough roads may cause false misfire detection. The rough road(acceleration)sensor consists of a piezoelectric vibration pick up which detects vertical acceleration of the vehicle. The sensor signal is used by the PCM to determine the degree of vertical movement of the car, for example, on a bumpy road. Since this may also cause uneven engine running, the PCM uses the signal to distinguish the phenomenon from actual misfiring.
DTC Description
The PCM must monitor the engine for misfiring possibly caused by ignition coil defects or injector fails. If misfiring is detected, the PCM will identify the cylinder(s) that has(have) misfired and then calculate misfiring rate for a given duration. The DTC for Misfire (P0301 to 0304) is set as soon as the misfiring rate exceeds the limit which may result in damage to the catalyst or increase emissions. The PCM stores the individual DTC for a cylinder which has more than a 10% total misfire rate.
DTC Detecting Condition
Detecting Condition
Possible Cause
DTC Strategy
● Calculation of engine roughness
Faulty spark plugs, high-tension lead, or Ignition coil
● Incorrect valve timing
● Uneven compression
● Air leakage
● Improper Fuel pressure or dirty fuel.
● Blocked/Leaking injectors
● Leakage between cooling system and cylinder

Enable Conditions
● 85 < Mass air flow(mg/STK) < 230
● 512 < Engine speed(RPM) < 6500
● 10.2 < MAF gradient(mg/STK/Seg.) < 200
● 140 < Throttle gradient(°TPS/sec.) < 200
● Coolant temperature > 20°C if Start temperature < -7°C
● No rough road
● No relevant failure
● 11V< Battery voltage< 16V
Threshold Value
● Misfire= 12~54% within 200 rev.
Diagnosis Time
● 200 revolution or 3*200 revolution
Mil On Condition
● Immediate
Threshold Value
● Misfire > 2% within 1000 revolutions
Diagnosis Time
● 1000 revolution or 4*1000 revolution
Mil On Condition
● 2 Driving Cycles

Ignition Primary Coil (Ω)
Ignition Secondary Coil (kΩ)
0.58Ω ± 10%
8.8kΩ ± 15%

Component Location

General Description
The rear heated oxygen sensor is mounted on the rear side of the Catalytic Converter (warm-up catalytic converter) or in the rear exhaust pipe, which detects the catalyst efficiency. The rear heated oxygen sensor (HO2S) produces a voltage between 0V and 1V. This rear heated oxygen sensor is used to estimate the oxygen storage capability. If a catalyst has good conversion properties, the oxygen fluctuations are smoothed by the oxygen storage capacity of the catalyst. If the conversion provided by the catalyst is low due to aging, poisoning or misfiring, then the oxygen fluctuations are similar to signals from the front oxygen sensor.
DTC Detecting Condition
Detecting Condition
Possible Cause
DTC Strategy
● Voltage range check
●Open in signal harness
●Open in ground harness
●Contact resistance in connectors
●Faulty Heated O2 Sensor(HO2S)
Enable Condition
● O2 Sensor operative readiness
● O2 Sensor preheating and full heating phases finished
● No relevant failure
● 10V < Battery voltage < 16V
Threshold Value
● 0.37 < Downstream O2 Sensor voltage < 0.49V & sensor element resistance > 60kΩ
Diagnostic Time
● 30 sec.
Mil On Condition
● 2 Driving Cycles

Schematic Diagram
Signal Waveform

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Regards Greg
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