Fix P0133: Mechanic’s solutions that WORKED

P0133 O2 Sensor Circuit Slow Response (Bank 1, Sensor 1) indicates a problem with your upstream oxygen sensor on Bank 1. Either the sensor is not working or there is another issue in air/fuel mixture that is confusing the O2 sensor.

P0133 quick overview

Meaning	O2 Sensor Circuit Slow Response (Bank 1, Sensor 1)
Is it serious?	Yes, it should be addressed to prevent potential damage to the catalytic converter.
Possible causes	– Faulty O2 sensor/wiring – Air intake issue (filter, vacuum leak) – Exhaust leak – If Cat was replaced, they might swapped or damaged wires/connectors – Exhaust leak – Using aftermarket/universal sensor
How to diagnose?	Check O2 Sensor Connections: Ensure that the O2 sensor is connected properly. Inspect for Exhaust Leaks: Examine the exhaust system for leaks that could affect O2 sensor readings. Test O2 Sensor Response: Use a scan tool to monitor O2 sensor response; a slow response may indicate a faulty sensor. Check for Vacuum Leaks: Vacuum leaks can impact air-fuel mixture; inspect for any leaks in the intake system. Examine Catalytic Converter: Assess the condition of the catalytic converter; a faulty converter can trigger this code. Verify Fuel System: Check the fuel system for proper pressure and delivery. Inspect Wiring and Connectors: Ensure that wiring and connectors related to the O2 sensor are in good condition. Consider O2 Sensor Replacement: If all else checks out, consider replacing the O2 sensor, preferably with an OEM part.
What is an Oxygen sensor?	Oxygen sensor measures amount of air in your exhaust to adjust air/fuel mixture for maximum efficiency.

What is “slow response”?

This code suggests that the upstream (pre-catalytic converter) oxygen sensor on bank 1 is not responding as quickly as expected to changes in the air-fuel mixture.

Which sensor is Bank1 sensor 1?

The upstream sensor is sensor 1 located before the catalytic converter. Bank 1 means exhaust system for engine head 1. Read the full Bank1 vs Bank2 explanation.

How to fix p0133 – learn from mechanics

I looked into P0133 service cases at iatn.com website where in the private forums the mechanics seek advice from other mechanics. I looked up a bunch of cases to help you decide how to fix your issue and see what would a real mechanic do to solve it.

Case #1: Saturn

Vehicle Information:
1996 Saturn SL1
Engine: 1.9L

Issue + Repair History:

• Intermittent MIL (Check Engine) light reported by the customer.
• Previous repair: O2 sensor replaced 2,000 miles ago.
• No drivability complaints.

Additional Information:

• Manifold Vacuum Readings:
  • Idle: 18 in.
  • Cruise: 18 in.
• Diagnostic Trouble Code (DTC): P0133 (O2 sensor slow response).
• The owner took steps to address the MIL light issue, including O2 sensor replacement and additional maintenance, with the final outcome pending further observation.

Suggestions from the Community:

• Recommendation for a factory O2 sensor.
• Mention of a potential thermostat (T stat) issue.

Final Fix:
The O2 sensor was replaced with an OEM part. The thermostat (T stat) was also replaced, and the coolant system was flushed. The issue was resolved.

Case #2: Volvo

Vehicle Information:
1996 Volvo 850 Turbo
Engine: 2.3L

Issue + Repair History:

• MIL (Check Engine) light with code P0133 (O2 sensor slow response).
• The O2 sensor had been previously replaced, providing only a temporary fix.

Suggestions from the Community:

• Check for air or exhaust leaks.
• Investigate potential compression issues.
• Examine the fuel system for problems.
• Consider the possibility of defective O2 sensors.

Final Fix:
A small exhaust leak near the catalytic converter was identified and repaired. After the repair, the car was test-driven, and all readings appeared normal. The issue was resolved.

Case #3: Chevrolet

Vehicle Information:
2006 Chevrolet Cobalt LS
Engine: 2.2L

Issue + Repair History:

• MIL (Check Engine) light with codes P0030 (O2 sensor heater control circuit) and P0135 (O2 heater circuit B1S1).
• Aftermarket O2 sensors were replaced due to an identified heater circuit short.
• The vehicle returned with MIL on, now showing code P0133 (O2 signal slow, Bank 1 Sensor 1).

Suggestions from the Community:

• Initial testing suggested a possible shorted O2 sensor.
• Performed tests on the low signal circuit, found 3 ohms (within acceptable range).
• Concerns were raised about potential damage to the PCM due to the shorted O2 sensor.

Final Fix:
After further testing and concerns about the performance of aftermarket O2 sensors, the aftermarket sensors were replaced with factory OEM O2 sensors. The issue was resolved.

Case #4: Ford

Vehicle Information:
1998 Ford Explorer XLT
Engine: 5.0L

Issue + Repair History:

• MIL (Check Engine) light with code P0133 (O2 signal slow, Bank 1 Sensor 1).
• Both O2 sensors were initially replaced.
• MIL light returned after a week with the same code.

Suggestions from the Community:

• Suggestions included checking for a vacuum leak in the intake system, inspecting the air filter, and verifying the exhaust differential pressure sensor.

Final Fix:
A vacuum leak in the intake system was repaired, and the severely clogged air filter was replaced. After these repairs, no recurrence of the issue was reported by the customer.

Case #5: Honda

Vehicle Information:
2000 Honda Civic LX
Engine: 1.6L

Issue + Repair History:

• Check engine light on with codes P0132, P0133, P0420.
• Bank 1 Sensor 1 (B1S1) was replaced with a Bosch sensor, and the rear O2 sensor showed a clean wave.
• The exhaust manifold (with a built-in catalytic converter) was replaced due to a small crack.
• Codes P0133 and P0420 persisted after replacing the exhaust manifold.
• B1S1 was replaced with an OE Honda sensor, and new spark plugs were installed.
• A pending code P0133 persisted despite clearing codes.

Suggestions from the Community:

• Rechecked basics (fuel pressure, MAP, TPS, CTS).
• Discovered that the connectors for O2 Sensor 1 (S1) and O2 Sensor 2 (S2) were swapped during installation.
• Mentioned that the digital signal to the O2 sensor was for the heater, with a normal wave type on both sensors.

Final Fix:
The connectors for O2 Sensor 1 and O2 Sensor 2 were swapped, leading to the issue. Reconnecting the connectors correctly resolved the problem, and the vehicle passed the drive cycle with no further codes.