P0138 Code – What Does It Mean & How To Fix It

This is one of the most frequent OBD2 trouble codes. Read the full article below to know what it means, how to fix it, and what other codes may show related to it.

Causes

There’s three potential causes that will trigger this trouble code. Here’s what to check first when you go to troubleshoot the code:

• The engine control module (ECM) registers a spike in voltage above 1.2 volts due to an O2 drop in the exhaust stream. This signals a “lean” fuel condition in the ECM, and it attempts to compensate by altering the fuel/O2 mix..
  
• The ECM registers a voltage spike and stores the code before flipping on your Check Engine Light. This could mean a sensor failure or a fuel/O2 mixture problem.
  
• The ECM is compensating for the high voltage issue by using other engine O2 sensors to alter fuel injector output and balance the fuel/O2 mixture.

Symptoms

How will you know your ECM is coding a possible P0138? Here’s the typical symptoms of the problem: 

• Your engine may run lean during sensor testing, leading to misfire and hesitation on startup or acceleration
• .The Check Engine Light will be illuminated, and you should always pull codes when that indicator lights up on your dashboard.
  
• Your engine may have air intake and fuel system problems depending on the root cause of the sensor voltage spike and subsequent “rich” fuel/O2 mixture.

Diagnosis

Let’s talk about finding out what caused your P0138. Here’s a step-by-step guide to troubleshooting this code.

• Start by scanning trouble codes and documenting the freeze-frame data. Now clear the codes and fire the engine up to verify it’s not a one-off reading. Here's a great one we use daily.
• When you connect the meter the second time, monitor O2 sensor data to verify the voltage is jumping and dropping faster on Bank 1, Sensor 2 compared to the other O2 sensors.
• Next, it’s time to do the obligatory wiring check. Make certain all wires, insulation and connectors are intact and free of corrosion.
  
• After your wiring check, inspect the O2 sensor itself for both fluid contamination and physical damage.
  
• Verify there are no leaks in your exhaust system. You don’t necessarily need a smoke test to check this out, but if you have the equipment or a home kit it’s probably worth checking out.
  
• Check your manufacturer’s maintenance manual for any other diagnostic steps or know issues with the exhaust system on your vehicle. There may be a known issue and quick fix or an ECM firmware update that needs to be installed.

Common mistakes

There’s a caveat for comparing sensor voltage between Bank 1, Sensor 1 and Sensor 2 to diagnose a P0138: both sensors should be operating at nearly identical voltage, but sensor 2 should indicate a lower O2 reading since the catalytic converter burns off the excess fuel and O2. If you aren’t seeing the O2 differential, there may be another root cause.

Many DIYers and some technicians also fail to check Sensor 2 for oil or coolant contaminants from any engine leaks, and that will create low O2 readings even if the sensor is functioning properly. Another potential problem that’s easy to overlook is a damaged catalytic converter. If it’s clogged with carbon or leaking air, it can cause erratic sensor readings and trigger a P0138.

How serious is this?

The voltage output from the O2 sensor may be due to the exhaust catalyst being broken apart, which can cause the O2 sensors to give high output voltages. A broken catalytic converter is an expensive fix and will definitely cause serious problems with your fuel system.

Your vehicle will also fail its state emissions test until you correct the problem. Additionally, your vehicle ECM may not control the fuel-to-air ratio of the engine properly if you have a broken sensor, leading to a clogged catalytic converter or fouled sparkplugs due to excessive carbon buildup. That way lies breakdowns, poor fuel economy and potential permanent engine damage.

What repairs can fix the code?

• Swap out Sensor 2 with a new O2 sensor (fast and moderately expensive)
• Repair/replace wiring or connectors to the O2 sensor for Bank 1, Sensor 2 (fast and cheap)
• Replacing the catalytic converter in front of the sensor (an expensive repair)
• Repairing a leaking injector (potentially an expensive fix)

Related codes

None listed.

Conclusion

Like many fuel and exhaust system issues, this is a problem that can creep its way up from a quick fix to a hefty repair bill. As soon as you see your check engine light come on, read the trouble codes on your own OBD-II scanner or take it into the shop right away. A quick fix at the beginning can save you significant time, money and headaches down the road.