What Does Engine Code P0140 mean?
- P0140 Technical Definition: Oxygen Sensor Circuit No Activity Detected (Bank 1 Sensor 2)
- P0140 Meaning: Downstream oxygen sensor is not working properly
- Most common cause: Faulty downstream oxygen sensor or wiring
- Risks for the engine/driver: LOW Car is still safe to drive but you should get it repaired soon
- Emissions severity: HIGH. The car won’t pass emissions testing
- Estimated repair time: 1 day
- Estimated repair cost: $100-200
The engine code P0140 is stored in memory when the Electronic Control Unit (ECU) senses poor to no activity in the oxygen sensor corresponding to engine bank #1 (the bank of cylinders where cylinder #1 is located). The sensor 2 tag indicates the post-catalytic converter O2 sensor.
The Oxygen Sensor (also known as O2 sensor) is in charge of reporting the ECU the amount of residual oxygen present in the exhaust gases. The O2 information is in turn used by the ECU to determine if the fuel delivery was too high (rich condition) or too low (lean condition).
In modern vehicles equipped with an A/F ratio sensor (still called O2 sensor by most technicians), the ECU counts with more information regarding fuel efficiency and hence adjusts fuel delivery with a higher degree of accuracy. The information coming from the O2 sensor is the main feedback used by the ECU to maintain the stoichiometric ratio of 14.7 parts of air for each part of fuel, that’s why O2 sensor is so important for the emissions system.
Besides the main O2 sensor (called sensor 1) the ECU also incorporates a second and in some cases a third O2 sensor after the catalytic converter. The purpose of the additional O2 sensors is to determine the efficiency of another key component of the emissions system: the catalytic converter (CAT).
To calculate the catalyst efficiency, the ECU compares the values of both oxygen sensors in real-time. The expected result is a “cleaner” reading in the downstream O2 sensor. When the upstream sensor (sensor 1) is responding as expected but the downstream sensor (sensor 2) is returning a constant value then the ECU assumes there is a problem.
The P0140 code is set when the ECU detects no activity (no variations/constant value) in the post-converter oxygen sensor.
5 Common Symptoms When Code P0140 is Present
The most common symptoms of data trouble code P0140 are:
- Check Engine Light Lit
- Increased emissions
- Lower fuel economy (due to ECU not being able to calculate fuel ratio accurately)
- Rough idle and running
- Bad overall performance, especially when accelerating
Possible Causes For Error Code P0140
The most common causes of data trouble code P0140 are:
- Damaged Oxygen sensor wiring (open, shorted, burnt)
- Damaged Oxygen sensor connector (loosely, corroded, disconnected or bent pins)
- Bad downstream oxygen sensor
- Dirty or faulty Mass Air Flow Sensor causing error code P0101.
How To Diagnose The Engine Code P0140
For the purpose of this article, it’s assumed that you have a basic knowledge of safety precautions while working on your vehicle.
IMPORTANT: the diagnostic procedures listed below assume you only have the data trouble code P0140 present. If you also have any other code related to the ignition system, emissions system or fuel system then you should start diagnosing them first.
Always refer to the appropriate OEM literature when possible. Original manufacturer diagnostic procedures should always have precedence over a generic workflow.
That said, let’s start the diagnostic process!
1. Preliminary steps
To discard a possible intermittent DTC condition, you’ll need to clear the ECU memory and complete a driving cycle.
- Read data trouble codes and take note of them.
- Clear data trouble codes memory.
- Perform a driving cycle (at least 5-10 minutes).
In case the Check Engine Light stays off then you may have an intermittent problem. If the light lit during your driving cycle then continue with the diagnostic process.
2. Visual Inspection
> Oxygen Sensor wiring and connector: perform a meticulous visual inspection of the downstream O2 sensor wiring and connectors. Look for burnt, damaged, corroded or deteriorated wires, also unplug the sensor and look for bent terminal pins, loosely connections, corrosion or any other possible indication of a bad connection.
> Exhaust pipe: any leak in the exhaust, especially before the downstream oxygen sensor will cause a false O2 sensor output. This may be interpreted by the Electronic Control Unit as a “no activity” condition. That’s why is important to verify the exhaust pipe condition and repair any leak including a bad exhaust manifold gasket leak.
Fix any wiring problem before continuing.
3. Scan Tool Tests
> Downstream Oxygen Sensor Test: start the engine to enter KOER mode. If your scan tool has a graphing ability turn it on. Graph engine RPM and O2 output simultaneously.
Accelerate the engine to 2000 RPM. You should see that the voltage is fairly stable. Now decelerate to idle speed. You should not see much variation in the O2 output.
Repeat the procedure several times.
If the downstream O2 sensor output closely resembles the upstream oxygen sensor output then you may have a bad catalytic converter.
Some manufacturers include special tests for the O2 sensors. Follow on-screen instructions to complete those tests. If you see no variation at all in the downstream O2 sensor then suspect a faulty sensor.
How To Repair Error Code P0140
Depending on the diagnostics results you may need to do the following:
- Repair the oxygen sensor(s) wiring and/or connectors.
- Repair the exhaust pipe
- Replace the downstream oxygen sensor
- Carry out further diagnostics on the catalytic converter
