|Trouble Code||Fault Location||Probable Cause|
|P0133||Heated oxygen sensor (HO2S) 1, bank 1 -slow response||Heating inoperative, wiring, H025|
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What Does Code P0133 Mean?
OBD II fault code P0133 is defined as “O2 Sensor Circuit Slow Response (Bank 1 Sensor 1)”, and is set when the PCM (Powertrain Control Module) detects that Heated Oxygen Sensor #1 is not operating within predefined parameters. Note that “Slow Response” in the definition refers to the time taken by the sensor to react to changes in the composition of the exhaust gas. “Oxygen Sensor 1” refers to the oxygen sensor upstream of the catalytic converter, and “Bank 1” refers to the bank of cylinders that contains cylinder #1. This distinction is only made on engines that have two cylinder heads.
Oxygen sensors with the #1 designation have the purpose of analysing the exhaust gas before it enters the catalytic converter. In terms of operating principles, a #1 oxygen sensor measures the amount of oxygen in the exhaust stream, based on which it generates a signal voltage that is sent to, and used by the PCM to make adjustments to the fuel trim to ensure that the ideal ratio between air and fuel (14.7 parts of air to 1 part of fuel) is maintained at all times, and under all operating conditions.
In an engine that is in perfect mechanical condition, and on which there are no misfiring, lean-, or rich running conditions, vacuum and/or exhaust leaks, or other defects, a change in the throttle position produces an almost immediate change in the amount of oxygen in the exhaust stream. The #1 oxygen sensor is designed to register this change near instantaneously by means of a change in the generated signal voltage; if this does not happen and the sensor takes longer than about 100 milliseconds to react after the PCM has entered closed loop operation, the PCM registers the slow reaction, stores a trouble code, and illuminates a warning light.
Unlike older narrow-band oxygen sensors, modern broadband heated oxygen sensors have the ability to control the air/fuel mixture right from the moment of start-up in sub-zero temperatures all the way to wide-open-throttle conditions in the hottest weather without significant lags in reaction times, which reduces emissions considerably. Note however that while response times vary between applications, the differences are not great. Nevertheless, it is recommended that the manual for the application being worked on be consulted for the correct response time, and other values such as control circuit /sensor resistance and reference voltage.
The panel of images below shows the most common types of fouling that can damage an oxygen sensor permanently. Oil fouling, which is not shown here, leaves a sticky, tarry residue on the sensor. If fouling by silicone-based compounds is found, cease the use of all aftermarket oil and fuel additives immediately, since some aftermarket additives that contain silicone-derived compounds can eventually destroy catalytic converters as well.
What are the common causes of code P0133 ?
Oil fouling that inhibits the sensing ability of oxygen sensors is probably the most common cause of code P0133 on applications that are known (notorious?) for excessive oil consumption, such as BMW, AUDI, VW, and Mercedes-Benz. Other possible causes could include the following-
- Defective oxygen sensors.
- Defects and problems in the heater control circuit that prevent the heating element from reaching full operating temperature.
- Large exhaust leaks that contaminate oxygen sensors’ reference air.
- Vacuum leaks that allow unmetered air to enter the engine.
- Low fuel pressure that causes under fuelling. Note that fuel system related codes will likely be present.
- Damaged, burnt, shorted, or corroded wiring and/or connectors.
- Open circuits. Blown fuses are common.
- Failed, or failing PCM. Note that this is a rare event, and the fault must be sought elsewhere before any controller is replaced.
What are the symptoms of code P0133 ?
In a few cases, there may be no noticeable symptoms other than the presence of a stored trouble code and an illuminated warning light. However, this condition is rare, and it is more likely that the following symptoms will be present-
- Engine may run rough at idle or not idle at all.
- Hesitation upon acceleration is common.
- Increased fuel consumption.
- Frequent or unpredictable engine stalling.
- In extreme cases, black smoke may be visible from the tail pipe.
NOTE: Not all of the above symptoms will always be present on all applications. Some symptoms may also vary in intensity between different applications.
How do you troubleshoot code P0133 ?
NOTE #1: Diagnosing code P0133 requires that the engine be in perfect running condition. If any other emission control, air/fuel metering, or misfiring codes are present along with P0133, these codes must be diagnosed and resolved in the order in which they were stored before attempting to diagnose P0133. Failing to do this will almost certainly result in a misdiagnosis.
NOTE #2: In order to save time and a possible misdiagnosis, clear all codes and rescan the system after each step in this procedure.
NOTE #3: In many cases where code P0133 is present, and especially on applications that are known for their excessive oil consumption, the problem can be resolved simply by cleaning the MAF (Mass Airflow Sensor) element with an approved cleaner. Oil accumulations on the heated element of MAF sensors can cause fuel metering issues and codes, of which code P0133 is one. If cleaning the MAF sensor element does not resolve the issue, proceed with the steps below.
Record all codes present, as well as all available freeze frame data. This information can be of use should an intermittent fault be diagnosed later on.
If there are other codes present along with P0133, diagnose and resolve them all before attempting a diagnosis of P0133.
When all other issues have been resolved, perform a thorough visual inspection of all associated wiring. Consult the manual on the location, function, routing, and color-coding of each wire in the control circuit, and look for damaged, burnt, shorted, or corroded wiring and connectors. Make repairs as required.
NOTE: It is common to find wiring burnt by hot exhaust components. If this is found, make sure to secure repaired wiring away from hot components to prevent a recurrence of the problem.
If no visible damage to wiring is found, perform reference voltage, ground, continuity, and resistance checks on all associated wiring. Compare obtained readings with those stated in the manual, and make repairs as required to ensure that all readings fall within specifications. Note that the oxygen sensor itself forms part of the control circuit, and as such its resistance must be tested as well. Deviations in resistance of more than about 10% to either side of the stated value will affect the operation of the sensor.
Be sure to disconnect the sensor from the PCM before commencing resistance and continuity checks to prevent damage to the controller.
NOTE #1: Pay particular attention to both the oxygen sensor’s heater control circuit, and the signal continuity/resistance of the signal wire between the sensor and the PCM during this step. The heater control circuit requires full battery voltage to heat the sensor element to the correct temperature, meaning that if the voltage is low for whatever reason, the sensor cannot operate as intended. The signal wire on the other hand, must have the correct resistance to be able to transmit the signal voltage efficiently.
NOTE #2: On same applications the heater control voltage is supplied by the PCM, and might therefore not be 12 volts. Consult the manual for the correct supply voltage BEFORE testing the heater control circuit.
NOTE #3: On most applications the heater control circuit is completed with a ground supplied by the PCM. Consult the manual on the correct procedure (KOER/KOEO) to establish the ground connection.
If all electrical values fall within specifications, remove the oxygen sensor from the exhaust, and inspect it for signs of damage or fouling. Undesirable deposits on the sensing element can seriously reduce the effectiveness of an oxygen sensor, or even render it completely useless, but remember that deposits cannot be removed without destroying the sensor. Refer to the image at the top of this guide to identify the most common types of fouling.
NOTE: There is little point in replacing the oxygen sensor if fouling is found. The underlying cause of the fouling must be found and resolved before this repair can be successful.
If the sensor is replaced, make sure all wiring is secured away from heat sources, clear all codes, and operate the vehicle with the scanner connected. Note that a change in throttle position must produce an immediate change in the signal voltage. On most applications, the signal voltage will be one volt or slightly less with a wide-open throttle, and about 100 millivolts or slightly more at idle.
The midpoint – around 400-, to 450 millivolts – represents a condition where the engine is under a steady load, and any change from this condition must produce an immediate change in the signal voltage. If the scanner can draw a graph of changes in the signal voltage, the rise and fall of the signal must be smooth, and without steps or “glitches” that represent lags in the sensor’s response time.
If the scanner can only display variations in the signal voltage as numbers, it might not be so easy to identify lags, since this type of scanner cannot update live data streams effectively. However, if the sensor and its control circuit are both working as intended, the code will not return.
Operate the vehicle for several drive cycles under varying conditions to allow the scanner to obtain a better data set. Record this data for future reference should the code return soon after the repair. If this happens, it is likely that there is an intermittent fault present, but be aware that this type of fault can sometimes be extremely difficult to find and repair. In extreme cases it might be necessary to allow the fault to worsen before an accurate diagnosis and definitive repair can be made.
Codes Related to P0133
- P0139 – Relates to “O2 Sensor Circuit Slow Response (Bank 1 Sensor 2)”
- P0145 – Relates to “O2 Sensor Circuit Slow Response (Bank 1 Sensor 3)”
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