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2000 VW Jetta P0135 and P0134 O2 Sensor Codes: Causes, Diagnosis, and Fan Circulation Issues

Model: 2000 VW Jetta Fault Code: P0134 Posted: 2015-01-18 21:40

Hello everyone, My check engine light came on recently, and after scanning the system, I found two diagnostic trouble codes: P0135 and P0137. Both are related to oxygen (O2) sensors. I'm wondering whether this is simply a case of replacing the upstream O2 sensor in Bank 1, or if there might be another underlying issue at play. In addition, I recently replaced the thermostat. Since then, I've noticed that the cooling fans are not activating — even though the engine heats up properly and doesn't overheat. I can feel the upper radiator hose getting hot when the engine reaches operating temperature, but I don’t observe any noticeable coolant flow or fan operation. I’ve read online that air trapped in the cooling system could be a cause, and I've tried several standard methods to purge the system (like bleeding it), but no improvement has been seen. My question is: Could this lack of fan activation and poor coolant circulation be related to the P0135 and P0134 codes? I'm mechanically inclined and have a good understanding of basic car systems, but troubleshooting — especially diagnosing electrical circuits or fluid dynamics — isn't my strongest area. Any insights or suggestions would be greatly appreciated. Thanks in advance! Best regards, Rob

Related fault codes
P0134P0135
Comments (2)
Anonymous 2015-01-19 08:05

For a 2000 VW Jetta, it's common for the O2 sensor to degrade over time, especially in older vehicles — so P0135 and P0134 are likely due to a faulty upstream O2 sensor. However, before replacing the sensor, verify the wiring harness, connectors, fuses, and power supply. A poor electrical connection can mimic O2 sensor faults. Ensure you installed the correct temperature-rated thermostat as specified by VW. Also, confirm that the cooling system has been properly 'burped' (air purged) after a fluid change or repair. If the engine reaches operating temperature without overheating, coolant circulation is likely occurring — but fans may not activate due to faulty wiring or sensor issues. Check for adequate heat transfer into the cabin as well; if the interior isn or feels cold, it could indicate poor cooling system performance.

Anonymous 2015-01-20 12:39

Thanks for the helpful input. I've already confirmed that the correct temperature-rated thermostat was installed — and since the engine doesn’t overheat, I assumed coolant circulation was working. I’ve tried several methods to purge air from the system, but still no fan activation. When the engine warms up, I can feel the upper radiator hose is hot, but there’s no noticeable pressure or flow. I’m now leaning toward the O2 sensor being faulty — especially since the fans aren’t running and the codes point directly to upstream sensors. Still, I’ll go through the wiring and connectors next to rule out any electrical faults. It's frustrating because I didn't want to spend time on a wiring issue only to find it was just a sensor replacement. Thanks again for the advice!