please help if you are a hyundai tech?pretty long.?

You did not specify if it the 2.4L 4 cylinder or the 2.7L V-6? Thank you for the additional information! The first thing that I would check is the sensor that they installed for the wrong sensor or folded over pins, also check the air intake pipe for cracks, If this were a restricted converter problem it would not straighten up then you tromp it! The P0051 is for B2S1 O2 sensor Heater circuit (low voltage) and has nothing to do with lean or rich mixtures, This code indicates that the PCM detected an unexpected low voltage condition on the HO2S-21 heater control circuit for 20 seconds. And fuel control codes for the O2 sensors are P0131, P0133,etc Not P0311 (that is a misfire code for a V-12 engine!

It is typical description of Catalytic converter failure on V6 engines. You have 3 of them, one on each bank plus the one in the middle (under carriage). Most likely is the third one (undercarriage one). The only way to check this is to remove the exhaust system from manifold back and run engine (short term-about a minute) and see if the engine got power. Just be aware it will be loud. Rev up the engine and see if you can make the difference in engine power. If its got power, replace the 3rd manifold. Usually, P0051 code on O2 sensor does not indicate a faulty sensor. It tells computer that reading is very lean/rich and runs outside its parameters. A fault code related to a faulty O2 sensor is P0131, P0132 or P0133 (depending which one is defective). Correction made to fault codes.

****EDIT****

Here are some technical tips related to this particular fault code:

The normal operating temperature of the HO2S(Heated Oxygen Sensor) ranges from 350 to 850°C(662 to 1562°F). The HO2S heater greatly decreases the amount of time required for fuel control to become active. The ECM provides a pulse width modulated control circuit to adjust current through the heater. When the HO2S is cold, the value of the resistance is low and the current in the circuit is high. On the contrary, if the temperature in the resistor of the sensor rises, the current drops gradually.

DTC Description

ECM sets DTC P0051 if the ECM detects that the front HO2S heater control line is short to ground.

DTC Detecting Condition

Item

Detecting Condition

Possible Cause

DTC Strategy

● Driver Stage Check

● Related fuse blown or missing

● Open or short to ground in power supply or control harness

● Contact resistance in connectors

● Faulty HO2S

Enable Conditions

● Battery voltage>10V

● 1%

Threshold Value

● Short to ground

Diagnostic Time

● 20 sec.

Fail Safe

● Heater open loop control

MIL On Condition

● 2 Driving Cycles

Specification

HO2S Heater

Normal Parameter

18~28°C(64~82°F)

Heater Resistance

Approx. 3~4Ω

Component inspection:

1. Measure resistance between terminals 3 and 4 of the sensor connector (Component side).

Specification : 3~4Ω[at 18~28°C(64~82°F)]

2. Is resistance within the specification?

YES

▶ Check for poor connection between ECM and component: backed out terminal, improper mating, broken locks or poor terminal to wire connection. Repair as necessary and go to "Verification of Vehicle Repair" procedure.

NO

▶ Check HO2S for contamination, deterioration, or damage. Substitute with a known-good HO2S and check for proper operation. If the problem is corrected, replace HO2S and then go to "Verification of Vehicle Repair" procedure.

If none of these works, rear catalytic converter is clogged. Replace it.