I have Mazda 6 2006 - 4 cylinder. I got the PO137 code.

Directly on the exhaust coupling with the manifold.

If your vehicle has single exhaust, there are two oxygen sensors.

If your vehicle has dual exhaust, there are four oxygen sensors.

Code 137.....whats the brand of tool you are using?? Go online and download the pdf ..manual for that tool and then you will know what the code means..

P0137 is an O2 sensor low voltage code. bank 1 sensor 2. This could cause a rough start. The sensor that needs replaced is on the drivers side, after the catalytic converter

P0137 - Oxygen (o2) sensor circuit low voltage (Bank 1 sensor 2)
If you have more than one (1) sensor on Bank 1 then it will be the rear sensor. Change the o2 sensor. You may also want to check the following:

Repair exhaust leak near the rear sensor (if leak is found)

Check for restriction in catalyst and replace as necessary

Repair short, open, or high resistance on o2 signal circuit

Most likely you will just need to replace the sensor. Hope this helps you.

CODE P0421. DEFINITION-CATALYST EFFICIENCY LOW-BANK 1. EXPLANATION-THE OXYGEN SENSORS MONITOR THE CATALYTIC CONVERTERS ABILITY TO STORE OXYGEN. PROBLEM CAUSES- 1. CATALYTIC CONVERTER DEFECTIVE (FAILURE POSSIBLY DUE TO #2,3, OR 4) 2. ENGINE MISFIRE OR RUNNING CONDITION. 3. LARGE VACCUM LEAK 4. ENGINE OIL LEAKAGE INTO EXHAUST-VALVE GUIDE SEALS, PISTON RINGS. THE PCM HAS DETERMINED THAT THE WARM UP CATALYST EFFICIENCY FOR BANK 1 IS BELOW TRESHOLD FOR THE CURRENT ENGINE OPERATING CONDITION. (BANK 1 IDENTIFIES THE LOCATION OF CYLINDER #1, WHILE BANK 2 IDENTIFIES THE CYLINDERS ON THE OPPOSITE BANK).

replace O2 sensors, or your Catalytic Converter.

the 4 cylinder has only one oxygen sensor. this sensor is a plug in with two wires coming out of it. As the exhaust leaves the motor the o2 sensor tells the car what carbon is leaving the. The o2 sensor is located on the exhaust pipe directly leaving the motor

bank 1 is the bank of cylinders closest to the firewall. the sensor will be located along the exhuast from that side after the catylitic converter

P0137 / P0157-Oxygen O2 Sensor Circuit Low Voltage (Bank 1, Sensor 2) refers to the second oxygen sensor on Bank 1. P0137 means the O2 oxygen sensor's voltage remained low for longer than 2 minutes. This, is interpreted by the ECM as a low voltage condition and sets the MIL. Bank 1 Sensor 2 is located to the rear of the catalytic converter and should produce an output signal relative to oxygen storage capacity of the catalytic converter. This rear (sensor 2) sensor is less active than the signal produced by the front sensor. However, if the ECM senses the sensor is inactive, this code will set.

A code P0137 may mean that one or more of the following has happened:

Faulty o2 sensor Exhaust leak near the rear sensor
Plugged catalyst
Short to voltage on O2 signal circuit
High resistance or open on O2 signal circuit

Possible Solutions…
Replace faulty sensor
Repair exhaust leak near the rear sensor
Check for restriction in catalyst and replace as necessary
Repair short, open, or high resistance on o2 signal circuit

This code does not always mean the O2 sensor is low, but if u want to replace it here is the info to do that and the part number.

The sensor you want is the one in the exhaust pipe of the engine cylinder bank that has #1 cylinder in it, as you face the engine with the hood open the cylinder bank u want is the rear bank, u will need to jack the car up and get under it to remove the sensor, u may also need a special socket to remove it, the sensor is the one that is before the catalytic converter. part number is:DENSO Part # 2344068
REAR; Before Catalyst