P1150 – Air/Fuel Ratio Sensor Circuit Range/Performance Malfunction Bank 2 Sensor 1 (Toyota, Lexus)

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By Reinier (Contact Me)
Last Updated 2017-01-17
Automobile Repair Shop Owner
CodeFault LocationProbable Cause
P1150 P1150 – Air/Fuel Ratio Sensor Circuit Range/Performance Malfunction Bank 2 Sensor 1 (Toyota, Lexus)
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Manufacturer Specific Definitions for P1150

MakeFault Location
AudiHeated oxygen sensor (HO2S) 1, Bank 2 - implausible lambda control value
BmwMisfire During Start Cylinder 5
CitroenLack Of HO2S21 Switch - Adaptive Fuel At Limit
DaewooManifold Pressure Sensor
FordLack of Upstream Heated Oxygen Sensor Switch Adaptive Fuel Limit Bank 2
HyundaiBarometric Pressure Sensor Malfunction
Land RoverHeated oxygen sensor (H02S), upstream, bank 2 – range/performance problem
LexusHeated oxygen sensor (H02S) – RH front – range/performance problem
LincolnHeated oxygen sensor (H02S) 1, bank 2 fuel trim (FT), at limit
MazdaHeated oxygen sensor (H02S) – LH or front bank -front (6 cyl) – adaptive limit achieved
MercuryHeated oxygen sensor (H02S) 1, bank 2 -fuel trim(FT), at limit
NissanTurbocharger (TC) wastegate regulating valve – LH – circuit malfunction
PeugeotLack Of HO2S21 Switch - Adaptive Fuel At Limit
SaabHeated oxygen sensor (H02S) 1, heater- voltage high
ScionAir/Fuel Ratio Sensor Circuit Range/Performance Malfunction Bank 2 Sensor 1
SubaruHeated oxygen sensor (H02S) 1, heater- voltage high
ToyotaAir fuel sensor - California - RH front - performance malfunction
VolkswagenHeated oxygen sensor (HO2S) implausible lambda control value

Table of Contents

  1. What Does Code P1150 Mean?
  2. What are the common causes of code P1150?
  3. What are the symptoms of code P1150?
  4. How do you troubleshoot code P1150?
  5. Codes Related to P1150
  6. Get Help with P1150

What Does Code P1150 Mean?

SPECIAL NOTES: While OBD II trouble code deals specifically with the performance of the #1 (upstream of the catalytic converter) Air/Fuel Sensor on Toyota and Lexus applications, it must be understood that even though Air/Fuel Ratio Sensors performs the same function as conventional heated oxygen sensors on other applications, the chemical reactions and the electronic circuitry inside A/F ratio sensors are several orders of magnitude more complex than those found in heated oxygen sensors.

Note that although conventional heated oxygen sensors are increasingly being replaced by more sensitive and accurate A/F ratio sensors by almost all carmakers, the complexity of the operating principles of A/F Ration Sensors preclude a full description of how exactly these sensors work. For this reason (since this guide can only provide a brief description) the information presented here is intended for general informational purposes only, and should therefore NOT be used in ANY diagnostic procedure for code P1150 without making proper reference to the manual for the application being worked on. END OF SPECIAL NOTES.  

OBD II fault code P1150 is a manufacturer specific code that is defined by carmaker Toyota and by extension, Lexus, as “Air/Fuel Ratio Sensor Circuit Range/Performance Malfunction Bank 2 Sensor 1. “ Sensor 1” refers to the sensor installed upstream of the catalytic converter, while on engines with two cylinder heads, “Bank 2” refers to the bank of cylinders that does not contain cylinder #1. On Toyota and Lexus applications, this code is set when the PCM (Powertrain Control Module) detects an abnormal voltage in the A/F Ratio sensor control or output (signal) circuits, given the current operating conditions.

The key to understanding how A/F ratio sensors work lies in not trying to compare their operation with the way conventional oxygen sensors work. Oxygen sensors of whatever type generate continuously changing signal voltages as a response to changes (the amount of oxygen) in the composition of the exhaust gas. While this allows the PCM to adjust short-term fuel trims to maintain the ideal air/fuel mixture for that application, the only thing the oxygen sensor tells the PCM is that the air/fuel mixture is either lean, or rich, but due to limitations in its design, it cannot tell the PCM by how much the air/fuel mixture is running rich or lean.

Thus, to maintain the ideal air/fuel mixture, the PCM has to switch the signal voltage from the oxygen sensor between rich and lean several times each second continuously, and then play “catch-up” by adjusting the injector pulse width (among other things), to keep the signal voltage to about 450 millivolts, which represents the ideal air/fuel mixture on most applications.

By contrast, Air/Fuel Ratio Sensors use a voltage differential between two independent measuring elements to maintain an almost perfect stoichiometric (14.7 parts of air to 1 part of fuel) air/fuel mixture for gasoline engines.

NOTE: Due to the many variables that operate in diesel engines, the stoichiometric air/fuel mixture for these applications vary, but suffice to say that Air/Fuel Ratio sensors work equally well on diesel engines.

In practice, an Air/Fuel Ratio Sensor on a Toyota/Lexus application will receive a constant, 3.3-Volt reference voltage from the PCM through one pair of wires, with the resulting signal voltage carried back to the PCM through another pair of wires. The signal voltage is generated by a complex relationship, or more accurately, the interaction between the two sensing elements, a sensing chamber between them, and the flow of oxygen molecules between the two elements. We need not go into the finer details here, but assuming that the sensor’s heating element is at 1 2000F (6480C), the sensor will either generate a signal voltage that is higher than 3.3 Volts, smaller than 3.3 Volts, or no voltage at all, which is the desired state, since it indicates that the air/fuel mixture is at the perfect stoichiometric (14.7:1, or Lambda = 1) ratio.

NOTE: The stoichiometric point is commonly referred to as “Lambda”, where Lambda = 1 represents the ideal 14.7: 1 air/fuel ratio. Lambda values greater than 1 represent a lean mixture, while Lambda values smaller than 1 represent a rich air/fuel mixture. For instance, if a capable scanner indicates a Lambda value of say, 1.17, the actual air/fuel ratio is 17.119, which is a lean mixture. Note that the factor is 14.7- thus, 1.17 multiplied by 14.7 = 17.119. The same “formula” applies to Lamda values smaller than 1- for example, Lambda at say, 0.94 multiplied by 14.7 = 13.818, which is a rich mixture.

The practical advantage of all of the above is that Lambda values smaller or greater than “1” can be expressed as a percentage by which the air/fuel mixture deviates from the ideal, 14.7: 1 stoichiometric standard, which is something that cannot be done with conventional oxygen sensors. Thus, “knowing” by what percentage the actual air/fuel mixture deviates from stoichiometric.

In response to the deviation from the stoichiometric point, a minute voltage differential is created between the two sensing elements in the Air/Fuel Ratio Sensor. This voltage differential is used by the PCM to adjust (among other things) the injector pulse width to either enrich, or lean out the air/fuel mixture to achieve the stoichiometric standard, at which point there is no current flow between the two sensing elements, thereby maintaining the air/fuel mixture within much narrower margins on either side of stoichiometric than is possible to do with conventional oxygen sensors.

Note that the reference voltage of 3.3 Volts from the PCM remains unchanged, regardless of how the actual air/fuel mixture changes. What does change however is that depending on the amount of oxygen (and therefore current), being exchanged between the two sensing elements, the signal voltage from the sensor to the PCM can flow in a positive or negative direction in much the same way a signal changes when the polarity of the signal generator is switched around. In this case, the “polarity” of the “signal generator” is analogous to whether the air/fuel mixture is rich or lean, relative to the stoichiometric point.

The image below shows a simplified schematic of the basic operation of a typical A/F ratio sensor.  Note that A/F ratio sensors and conventional oxygen sensors are NOT interchangeable, despite the similarities in their appearance.


What are the common causes of code P1150?

Some common causes of code P1150 could include the following-

  • Damaged, burnt, shorted, disconnected, or corroded wiring and/or connectors, especially on the sensor’s heater circuit
  • Defective heater circuit relay (where fitted)
  • Defective or contaminated sensor sensing elements. Likely contaminants are engine coolant, sulfur or zinc from engine oil, and/or silicone-based gasket sealers
  • Exhaust leaks
  • Engine vacuum leaks. Note that this condition is likely to be indicated by one or more dedicated codes
  • Excessive fuel pressure. Note that this condition will be indicated by a dedicated code
  • Misfires on one or more cylinders. Note that this condition will definitely be indicated by dedicated misfire-related codes
  • 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 P1150?

Common symptoms of code P1150 are much the same as the symptoms of a bad oxygen sensor, and could include the following-

  • Stored trouble code(s), and an illuminated warning light
  • Increased fuel consumption and higher-than-acceptable levels of exhaust emissions
  • Power loss to varying degrees, depending on the exact nature of the problem
  • Vehicle may not pass an emissions test

How do you troubleshoot code P1150?

SPECIAL NOTES: As a practical matter, it is rare to encounter code P1150 without other Air/Fuel Sensor related codes being present as well. However, while P1150 refers to a range/performance issue of the A/F ratio sensor on Toyota/Lexus applications specifically, there are many generic OBD II codes that could indicate the cause of the range/performance problem. Typical generic codes that may be encountered on some Toyota (but somewhat-less often on Lexus) applications could include P0036, P0037, P0038, P0042, P0043, P0044, P0050, P0051, P0052, P0056, P0057, P0058, P0062, P0063 and P0064. Note that these codes generally indicate malfunctions of, or failures and defects in the A/F ratio sensor’s heater circuit, which is arguably the most common reason why A/F ratio sensors sometimes do not perform as expected.  

Typical generic codes that indicate failures of or defects in, the A/F ratio sensor itself could include any generic code(s) from P0130 through P0167, although one or more additional Toyota/Lexus P1XXX codes (most often P1135 and/or P1155 (relating to heater circuit/element issues)) may be present as well. Note that operating the application on low fuel levels for extended periods or actually running out of fuel will also set code P1150.

In light of the above, it is vitally important that when code P1150 is present on any Toyota/Lexus application, that all fault codes and available freeze frame data be recorded, and that all codes are researched and resolved in the order in which they were stored. Failure to do this will almost certainly result in a misdiagnosis, and the almost certain (unnecessary) replacement of the A/F ratio sensor, which costs several hundred dollars. END OF SPECIAL NOTES.

WARNING: Even though A/F ratio sensors are extremely tough and durable, their design and construction makes them highly vulnerable to damage caused by over tightening, or the use of excessive force during removal. However, the very high operating temperatures (1 2000F / 6500C) of these sensors often cause their threads to become “welded” to the mating threads in the exhaust, which almost always requires much more than the recommended tightening torque of 30 ft/lbs to overcome during removal. For this reason, ALL avenues short of removing the sensor from the exhaust MUST be explored when diagnosing this code BEFORE an attempt is made to remove the sensor from the exhaust system. Note that removal of the sensor requires the use of a special slotted socket, which is available from specialist tool stores or auto some parts stores.

NOTE #1: While testing the operation of an A/F ratio sensor has some similarities with the procedure used for testing conventional oxygen sensors, the biggest differences involve the interpretation of obtained results, and the fact that most non-professional mechanics do not possess either the skill or knowledge required, or have access to the specialized equipment required to perform more than a few basic tests and checks to diagnose this code. However, the few diagnostic/repair steps outlined here should enable most non-professional mechanics to diagnose and/or repair code P1150 on most Toyota/Lexus applications successfully.

NOTE #2: All A/F ratio sensors use a “tuning chip” (resistor) that is incorporated into the electrical connector a means to enhance their accuracy even further. This resistor MUST be checked and tested during ANY diagnostic procedure for code P1150, or for that matter, any other A/F ratio sensor code. Note also that the resistance value of this resistor varies between applications, so if either, or both the resistor and the sensor must be replaced, be sure to replace the resistor with the CORRECT resistor for that application to avoid a recurrence of this, or other codes.

NOTE #3: Be aware that although diagnosing this code is not an overly complicated affair, doing so requires the use of an enabled or capable code reader/scanner, because the voltage differences that result from changes in the oxygen content of the exhaust stream occur extremely rapidly, and are very small. This means that even the best clamp-on digital multimeters cannot register the changes reliably (or at all), and besides, due to the design/nature of the sensors’ wiring, the actual signal wire needs to be cut to “splice” the instrument into the system. Doing this is NOT a good idea, so if a suitable scanner or oscilloscope is not available, refer the vehicle to a specialist repair shop for professional diagnosis and repair.

NOTE #4: Depending on the capabilities of the scanner, the A/F ratio sensors’ output may be presented as a Lambda value, the actual air/fuel ratio, a fuel trim value, or a voltage value, where voltages displayed above the reference voltage (normally 3.3 Volts on most Toyota applications, or 2.6 Volts if Bosch sensors are fitted), indicates a lean mixture, while displayed voltages lower than the reference voltage indicates a rich mixture. Also, note that on some older Toyota applications, A/F ratio sensor data may be presented as “oxygen sensor” data. This is because many early scanners could not read A/F ratio sensor data, so these early OBD II systems the data was converted to simulate oxygen sensor data to comply with the OBD II regulations of the time.

Step 1

Record all fault codes present, as well as all available freeze frame data. This information can be of use should an intermittent fault be diagnosed later on. Refer to the SPECIAL NOTES at the top of the Troubleshooting section of this guide for details on codes that are likely to accompany P1150.

NOTE: Particular additional codes to look for are those that indicate issues with the sensor heater monitor status, loop status, and coolant temperature. The presence of these codes will indicate whether or not some critical monitors have run and completed, which can sometimes contribute to code P1150 if they have not completed.

Step 2

Since it is possible to retrieve a constellation of codes, it is always a good idea to check for current and past TSB’s at this point. For instance, if code P0031 and/or P2238 is present on a 2.4L Toyota Camry, the only remedy is a replacement of the A/F ratio sensor, and having the PCM re-calibrated which in Toyota-speak, means having the PCM reprogrammed. There are other examples, so always research all codes found, or contact the dealer for details on active TSB’s.

Step 3

Since many, if not most A/F ratio sensor range and performance issues stem from failures of, or malfunctions in the heater control circuit, start the diagnostic procedure by consulting the manual for the application to determine the location, function, color-coding, and routing of the affected sensor’s wiring. Note that A/F ratio sensors on Toyota/Lexus applications have 5 or more wires.

Inspect all associated wiring and connectors for signs of damage, short circuits, open circuits, or other issues, such as corroded wiring and/or connectors. Make repairs as required.

Step 4

If no visible damage to wiring and/or connectors is found, consult the manual on the correct procedure to test the operation of the heater circuit. Although this process is largely the same as testing a heater circuit on a conventional oxygen sensor, the actual current draw on a Toyota/Lexus A/F ratio sensors varies both with the application, and the heater duty cycle in use on that application.

For instance, while Toyota/Lexus sensors typically draw about 75 Watts (app. 5-7 amps) for the first 20 seconds or so after starting a cold engine, this draw varies as the engine load and speed changes. This is normal and does therefore not necessarily indicate a fault condition, but be sure to consult the manual for detailed information on the sensor heater duty-cycle for that particular application. The manual will also provide information on how to check that the heater duty cycle works as intended for that particular application, so be sure to follow the directions EXACTLY to obtain the most accurate and reliable results. Refer to the NOTE below if significant deviations are found between the observed duty cycle, and the dusty cycle parameters stated in the manual.

NOTE: Be sure to check that none of the fault codes accompanying P1150 indicates a shorted or open heater circuit. If such codes are present, resolve them strictly in accordance with the recommended repair options stated in the manual before proceeding, but be aware that it might be necessary to remove the sensor from the exhaust. Refer to the WARNING at the top of the Troubleshooting section.

Step 5

Some A/F ratio sensor range/performance issues are caused by exhaust leaks that contaminate the sensors’ reference air, so if there is no damage to wiring and the sensor’s heater circuit is known to be good, inspect the exhaust system for even the tiniest leaks. Any and all exhaust leaks must be repaired before proceeding.

If the exhaust does not leak, all wiring is good, and the sensor itself appears to be undamaged, start the engine again and use the scanner to verify that the sensor has entered closed-loop operation, which happens when the PCM uses information from the A/F ratio sensor to control the air/fuel mixture.

Step 6

From this point onwards, much depends on the type, and capabilities of the scanner being used, as well as on how the sensors’ output is displayed on the scanner. If the sensors’ output is displayed as a Lambda value, refer to the note below to calculate the actual air/fuel ratio-

NOTE (Repeated from the Overview section): The stoichiometric point is commonly referred to as “Lambda”, where Lambda = 1 represents the ideal 14.7: 1 air/fuel ratio. Lambda values greater than 1 represent a lean mixture, while Lambda values smaller than 1 represent a rich air/fuel mixture. For instance, if a capable scanner indicates a Lambda value of say, 1.17, the actual air/fuel ratio is 17.119, which is a lean mixture. Note that the factor is 14.7- thus, 1.17 multiplied by 14.7 = 17.119. The same “formula” applies to Lamda values smaller than 1- for example, Lambda at say, 0.94 multiplied by 14.7 = 13.818, which is a rich mixture.

Note that apart from wiring issues, there are many other possible causes of a Lamda value that deviates from the stoichiometric point. Problems like excessive fuel pressure or engine vacuum leaks (among others) are common, so refer back to the list of accompanying codes to identify the most likely cause of the lean/rich mixture. Also, be sure to refer to the manual for detailed information on the causes that are most likely to produce code P1150 on that particular application before drawing any conclusions on the accuracy (or otherwise) of the displayed Lambda value.

Step 7

If the scanner displays a voltage value or the actual air/fuel mixture (expressed as a percentage) above or below stoichiometric, consult the manual on the correct procedure to follow to induce a change in the displayed value.

In most cases, this will involve using the scanner to command an increase in the injector pulse width by 25%, followed immediately by reducing the injector pulse width by 12.5%. The changes in the quantity of fuel injected will produce an almost immediate (within 1.1 second) change in the value displayed on the scanner, but the changes must be in directly related to the changes in the quantity of fuel injected.

For instance, a fully functional A/F ratio sensor will cycle between close to 3.0 volts (a rich condition), and close to 3.35 volts (a lean condition), provided the quantity of fuel injected changed by 25%, and 12.5% respectively. Assuming that the sensor is at the correct temperature, voltage fluctuations outside of this range can be taken as confirmation that the sensor is either defective, or that the sensor is contaminated with a substance that affects its operation.

If the sensor does not respond to changes in the air/fuel mixture, or if it takes more than about one second to respond, the sensor is defective, and it must be replaced.

Step 8

Sadly though, many, if not most scanners do not have the ability to control the injector pulse width as described in Step 7, which means that other methods of inducing a change in the reading is NOT reliable, with the possible exception of verifying that the sensor is either working or not, but without knowing how well it works- if it works.

For instance, injecting propane into the inlet duct to enrich the air/fuel mixture, or to snap the throttle open and closed is not reliable because there is no telling how much propane is required to enrich the mixture by 25%, or by how much the air/fuel mixture had been enriched when the throttle was snapped open- which means that the induced change in the displayed is largely useless, if not actually irrelevant. Similarly, there is no telling how large an artificial vacuum leak must be to mimic a 12.5% reduction in the quantity of fuel injected, which also means that the induced change in the displayed value by creating an artificial vacuum leak is also largely meaningless.

Step 9

In practical terms, the average non-professional mechanic has only two remaining options, the first being to obtain a high-precision gas analyzer to take accurate samplings of the exhaust gas at the tail pipe, and the second being to simply replace the A/F ratio sensor if it has not been replaced during the past 80 000, to 100 000 miles.

However, the first choice requires that the catalytic converter be in perfect working condition, which can be confirmed by following the directions in the manual to check the operation of the #2 (downstream of the converter) sensor. Note that this is a viable test method since Toyota has assigned an extraordinary amount of fuel control functions to the #2 sensor.

The last option available to most non-professional mechanics is simply to replace the sensor, but be aware that removing it from the exhaust to check for signs of contamination can damage the sensor, especially if it has not been removed before. In many cases, the female thread in the exhaust is destroyed in the process of removing the sensor, and while tapping out the thread might repair the damage, it often does not.

In some cases, it might be necessary to cut or drill the old bung from the exhaust and to weld in a replacement, (which requires special welding equipment), so this step is often best left to a specialist exhaust-repair shop.

  • P1133 – Relates to “A/F Sensor Circuit Response Malfunction (Bank #1 Sensor #1)”
  • P1135 – Relates to “A/F Sensor Heater Circuit Malfunction (Bank #1 Sensor #1)”

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