P1133 – HO2S Insufficient Switching Sensor 1 (Acura, Buick, Cadillac, Chevrolet, GM, Honda, Isuzu, Pontiac, Saturn)


By Reinier (Contact Me)
Last Updated 2017-01-17
Automobile Repair Shop Owner

CodeFault LocationProbable Cause
P1133 P1133 – HO2S Insufficient Switching Sensor 1 (Acura, Buick, Cadillac, Chevrolet, GM, Honda, Isuzu, Pontiac, Saturn)
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Manufacturer Specific Definitions for P1133

MakeFault Location
AcuraHeated oxygen sensor (HO2S) - RH front - insufficient switching
AudiHeated oxygen sensor (HO2S) 1, Bank 1 & 2 - heater control - circuit low
BuickHeated oxygen sensor (H02S) 1 -insufficient switching
CadillacHeated oxygen sensor (H02S) 1, bank 1insufficient switching
ChevroletHeated oxygen sensor (H02S) 1, bank 1 insufficient switching
CitroenHO2S Insufficient Switching Sensor 1
DaewooOxygen sensor (02S) 1, bank 1 – slow response
DaihatsuRear bump device abnormal
DodgeECT Sensor Circuit Interm Low Voltage
FordBank 1 Fuel Control Shifted Lean
GmcOxygen sensor (02S)/heated oxygen sensor(H02S) 1, bank 1 -insufficient switching
GmHO2S Insufficient Switching Bank 1 Sensor 1
HondaHeated oxygen sensor (HO2S) - RH front - insufficient switching
HummerHeated oxygen sensor (H02S) 1, bank 1 insufficient switching
HyundaiVapor Fuel Solenoid Malfunction
IsuzuHeated oxygen sensor (HO2S) - RH front (6 cyl) - insufficient switching
KenworthP1133 - WRAF heater CJ125 short-circuit to battery fault for heater diagnostic
LexusHeated oxygen sensor (H02S) – LH front- response malfunction
LincolnBank 1 Fuel Control Shifted Lean
MahindraTemperature Dependent Over Current Error For Egr Valve H-Bridge
MazdaFuel trim (FT) lean mixture – bank 1
MercuryBank 1 Fuel Control Shifted Lean
OldsmobileHeated oxygen sensor (H02S) 1, bank 1insufficient switching
PeterbiltP1133 - WRAF heater CJ125 short-circuit to battery fault for heater diagnostic
PeugeotHO2S Insufficient Switching Sensor 1
PontiacOxygen sensor (02S)/heated oxygen sensor(H02S) 1, bank 1 -insufficient switching
SaabHeated oxygen sensor (H02S) 1, bank 1 – insufficient switching
SaturnOxygen sensor (02S)/heated oxygen sensor(H02S) 1, bank 1 -insufficient switching
ScionAir/Fuel Ratio Sensor Circuit Response Malfunction Bank 1 Sensor 1
SubaruHeated oxygen sensor (H02S) 1 – voltage high
SuzukiHO2S11 Too Few Transitions
ToyotaHeated oxygen sensor (H02S) – LH front- response malfunction
VolvoHeated oxygen sensor (H02S) 1, bank 1 –   no signal
VolkswagenHeated oxygen sensor (HO2S) heater control - circuit low

What Does Code P1133 Mean?

OBD II fault code P1133 is a manufacturer specific code, and on some applications, this code is defined as “HO2S (Heated Oxygen Sensor) Insufficient Switching Sensor 1”, and is set when the PCM (Powertrain Control Module) detects an insufficient number of switching cycles between a rich running and a lean running condition in a period of time set by the manufacturer. Note that “sensor  1” refers to the oxygen sensor upstream of the catalytic converter.

The key to diagnosing and repairing code P1133 is to understand that oxygen sensors do NOT switch by themselves. A fully functional oxygen sensor only reacts, or “switches” from registering a lean-running condition, to registering a rich-running condition as a response to changes in the oxygen content of the exhaust stream. This need to “switch”, or generate changing signal voltages to reflect a change in the oxygen content of the exhaust stream derives from the fact that no engine can maintain the ideal 14.7 : 1 fuel ratio without “guidance” from the upstream oxygen sensors.

While early, unheated oxygen sensors had no effect on the short-term fuel trim of an engine until such time that they reached their optimum operating temperature of about 6000F, newer heated sensors reach this temperature in very short periods. In practice, this means that when an engine is started, the oxygen sensors are active almost immediately, which greatly reduces emissions.

Moreover, when an engine is started, the upstream oxygen sensor(s) registers whether the air/fuel mixture is lean or rich upon which, the sensor(s) generate the appropriate signal voltage that could be anywhere from about 0.1 volt, to about 0.9 volt. The midway point, about 0.45 volt, is an almost universal value that indicates that the engine is receiving the ideal air/fuel mixture. Be aware though that these values might be different between applications.

Nonetheless, when an engine starts, or soon after, the PCM starts a counter to count the number of times that the upstream oxygen sensor detects changes in the oxygen content of the exhaust stream in order to establish and maintain the correct air/fuel mixture from start-up (or soon after) when the engine is likely to run rich, to when the engine is significantly hotter, and therefore combusts the air/fuel mixture more completely.

The switching counter typically runs for about 100 seconds, during which time the PCM expects to “see” a specific number of “switches” between rich-, and lean running conditions per second. On multi-point fuel injected engines, this value is typically between 5-7 “switches” per second, with the PCM reacting to each “switch” in the signal voltage it receives by either increasing or reducing the injector pulse width to adjust the amount of fuel being injected into the engine.

Thus, when the PCM does not receive the amount of “switches” it expects to receive during the count, it will set code P1133, and illuminate a warning light. Note however that the time the counter runs, as well as the number of “switches” per second the PCM expects to receive varies between applications.

The image below shows typical types of contamination that can affect the switching times of an upstream oxygen sensor.


What are the common causes of code P1133?

The possible causes of this code are many and varied, and could include the following-

  • Damaged, burnt, shorted, disconnected, and/or corroded wiring and/or connectors
  • Open circuits
  • Defective oxygen sensor(s)
  • Stuck open EGR valve
  • Exhaust leaks that contaminate oxygen sensors’ reference air. Repair all exhaust leaks before attempting a diagnosis of P1133.
  • Unmetered air entering the engine
  • Defective, or oil-fouled MAF sensors on some applications
  • Misfires on one or more cylinders. Note that all misfire related codes must be fully investigated and resolved before attempting a diagnosis of P1133.
  • Failed, or failing PCM. Note however that this is a rare event, and the fault must be sought elsewhere before any controller is replaced.

What are the symptoms of code P1133?

In most cases, the only symptom that is likely to be present is a stored trouble code, an illuminated warning light, and an inability to pass an emissions test. Nonetheless, it is possible for other symptoms to be present, which could include the following-

  • Hard starting
  • Rough, or erratic idling under some conditions that may vary between applications
  • Increased fuel consumption over extended periods
  • Catalytic converter damage over extended periods
  • In extreme cases, some black smoke may be visible from the tail pipe

How do you troubleshoot code P1133?

NOTE #1: An accurate diagnosis of code P1133 requires that the engine be in perfect running condition. There should be no unmetered air entering the engine, no misfires present, and no exhaust leaks that could contaminate an oxygen sensor’s reference air. If any codes relating to air and fuel metering or the EGR (Exhaust Gas Recirculation) valve are present, these codes MUST be resolved in the order in which they were stored before attempting a diagnosis of P1133. Failure to do this will almost certainly result in a misdiagnosis, and hours of wasted diagnostic time.

NOTE #2: On some applications, most notably some BMW, VW, Mercedes-Benz, and Audi products, excessive oil consumption can foul oxygen sensors, and cause MAF (Mass Airflow) sensors to generate invalid signal voltages when they become contaminated with oil. Both conditions can set code P1133, so on these applications, a simple cleaning of the MAF sensor element with an approved solvent will often resolve this code. Moreover, the use of aftermarket MAF sensors can cause this code to be set, even in the absence of excessive oil consumption.

NOTE #3: Gasket sealers that contain silicone compounds can set this code, through silicone contamination of the upstream oxygen sensors’ sensing element. In fact, silicone fouling is the foremost cause of oxygen sensor failure, so before attempting a diagnosis of code P1133 on older engines, make sure that there are no silicone-based sealants/sealers present anywhere on the engine. The only remedy for silicone fouling of oxygen sensors is to remove all suspected or confirmed sealants/sealers on the engine to prevent a recurrence of the problem.    

NOTE #4: Always consult the manual for the application being worked on for detailed information regarding the time period that the “switch” counter runs, the minimum and maximum signal voltages the upstream oxygen sensors are expected to generate (typically between 0.1 volt and 0.9 volt), as well as the number of “switches” per second the upstream oxygen sensors are expected to make. Also, be aware that on some applications, the upstream oxygen sensors might be electrically opposite to the norm, with a low signal voltage indicating a rich-running condition, and vice versa.

Step 1

Assuming that no other codes are present, record the code, as well as all available freeze frame data. This information can be of use should an intermittent fault be diagnosed later on.

Step 2

Start the engine, and allow the PCM to enter closed loop operation. “Closed loop” operation commences when the PCM uses signal voltages from the oxygen sensor(s) to regulate short-term fuel trim adjustments.

With the scanner connected, open and close the throttle several times to produce changes in the exhaust streams’ oxygen content. The scanner will indicate the maximum and minimum signal voltages generated by the sensors; note the indicated values for future reference.

Step 3

If the scanner confirms that the oxygen sensor(s) do indeed not switch enough times, consult the manual to determine the location, routing, color-coding, and function of each wire in the affected circuit, and prepare to perform a through visual inspection of all associated wiring and connectors.

Look for damaged, burnt, shorted, disconnected, and/or corroded wiring and connectors. Make repairs as required, clear the code, and operate the vehicle under closed loop conditions with a scanner connected to monitor the working of the affected sensor(s) in real time.

Step 4

If no visible damage to wiring and/or connectors is found, prepare to perform ground, resistance, reference voltage, and continuity checks on all associated wiring, but be sure to disconnect the affected sensor(s) from the PCM to avoid damage to the controller.

All heated oxygen sensors have their own dedicated heater control circuits, so be sure to consult the manual to identify the heater control-circuit wire(s). Pay particular attention to the resistance of this circuit, since a sensor that does not reach full operating temperature can set this code because it does not work as intended. However, if the heater control circuit is the problem, it is likely that other codes will be present along with P1133.

Also, check the resistance of the signal wire from the sensor to the PCM very carefully, as an incorrect resistance can influence the switching count. Compare all obtained readings with those stated in the manual, and make repairs as required to ensure that all electrical values fall within specifications.

NOTE #1: On some applications the ground for the heater control circuit is supplied through contact with the vehicle frame, while on many others, this ground is supplied by the PCM. Consult the manual on the correct procedure (KOER/KOEO) to establish the ground.

NOTE #2: Be sure to route, and secure all wiring away from hot exhaust components. Heat damage to oxygen sensor control circuits is a common cause of a variety of oxygen sensor related codes, so place all wiring out of harm’s way.

Step 5

Since the affected oxygen sensor(s) form part of the circuit, be sure to test the internal resistance of each affected sensor as well, and replace those that do not comply with the manufacturer’s specifications.

NOTE #1: On some applications, it might be easier to test the sensors if they are removed from the vehicle. Whatever the case may be though, it is always wise to remove oxygen sensors from the vehicle to be able to inspect them for signs of contamination, or the presence of harmful deposits such as carbon. Be aware that if contamination or excessive carbon build-up is found such sensors cannot be cleaned or restored reliably. The only reliable remedy is replacement of the sensor(s).

NOTE #2: If evidence of contamination or carbon build-up is found, there is little point in replacing the sensors without correcting the underlying cause of the contamination or build-up. However, on applications that are known for their excessively high oil consumption rates, the only way to mitigate the frequency with which oxygen sensor related codes occur, is to replace the oxygen sensors more frequently, i.e., before oil contamination reaches the point where it destroys the oxygen sensors.

Step 6

After repairs to wiring, or sensor replacement(s) is complete, clear the code, and operate the vehicle in closed loop operation for at least one complete drive cycle to see if the code returns. If the code returns, it is likely as the resulted an intermittent fault.

Intermittent faults can sometimes be extremely challenging and time-consuming to find and repair, and in some cases, the fault may have to be allowed to worsen before an accurate diagnosis and definitive repair can be made.

Codes Related to P1133

  • P1134“HO2S Transition Time Ratio Sensor 1”, which code relates to the average amount of switches an upstream oxygen sensor makes from registering rich-to-lean-to-rich running conditions during the time the sensor is monitored. Note however that some manufacturers have assigned codes other than P1134 to this definition. Always consult the manual for the application being worked on for detailed information on the definitions of codes that affect that application.

Note also that some manufacturers have assigned definitions to code P1133 that do not relate to the number of times an oxygen sensor switches. Below are some examples-

  • Fiat – Oxygen sensor B1-S1
  • Lexus – Air/Fuel Ratio Sensor Circuit Response Malfunction (Bank 1 Sensor 1)
  • Mercedes – Bank 1 Fuel Control Shifted Lean (FAOSC)
  • Mercury / Oldsmobile/ Lincoln / Jaguar – Bank 1 Fuel Control Shifted Lean
  • Saab – O2S Circuit Low Voltage Sensor 1
  • Scion – Air/Fuel Sensor Circuit Response (Bank 1 Sensor 1)
  • Toyota – Air/Fuel Sensor Circuit Response Malfunction
  • Volkswagen /Audi / Volvo – O2 Sensor Heating Circuit Bank 1 + 2 – Sensor1 Electrical Malfunction

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