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2005 Chrysler 300C P1521 Code: Wrong Oil Viscosity Detected Despite Using 5W-20?

Model: Chrysler 300 Fault Code: P1521 Posted: 2019-11-21 15:02

I recently encountered a persistent trouble code P1521 on my 2005 Chrysler 300C with the 5.7L HEMI engine. I've maintained proper oil and filter changes over the past four years, consistently using 5W-20 synthetic oil as recommended. Despite this, the vehicle is showing a P1521 code — which typically indicates an incorrect oil viscosity or type. After researching, most sources suggest that 'wrong oil' is the primary cause of this code. However, I've used 5W-20 throughout my ownership and have never experienced any issues with oil quality or engine performance. This makes me highly skeptical about whether the issue lies in the oil itself or somewhere else in the system. I'm wondering if there could be a sensor malfunction — such as an oil viscosity sensor or oil sending unit — that's incorrectly interpreting the oil properties, even when using the correct specification. Given that this code is triggered by the Engine Control Module (ECM) based on multiple inputs, it may point to a faulty sensor or wiring issue rather than actual oil incompatibility. I'm seeking advice from experienced owners and technicians: Could P1521 be caused by a sensor fault instead of incorrect oil? What steps should I take to diagnose this code accurately — especially since the oil has been changed regularly with proper viscosity?

Related fault codes

P1521

Comments (10)

Anonymous 2019-11-21 20:49

Could the issue be related to an oil quality sensor malfunction? Some sources suggest a faulty sensor might trigger P1521 even when using correct oil type and viscosity.

Anonymous 2019-11-22 09:54

I’ve been experiencing this same issue. I’ll try replacing the oil quality sensor (if it exists) to see if that resolves the P1521 code.

Anonymous 2019-11-22 12:22

Note: While some forums refer to an 'oil quality sensor,' most Chrysler vehicles don't have a dedicated oil quality sensor. Instead, the ECM uses data from oil temperature, pressure, and sending units to estimate viscosity. It's possible one of these sensors is providing incorrect input, leading to P1521 detection.

Anonymous 2019-11-23 12:26

The P1521 code in a 2005 Chrysler 300C is typically triggered when the ECM detects an incorrect oil viscosity. Common causes include: incorrect oil type/viscosity, oil contamination, or sensor inaccuracies. First step: visually inspect wiring harnesses and connectors near the engine oil system for damage, corrosion, or loose connections. Since the vehicle lacks a dedicated oil quality sensor, the ECM relies on multiple sensors (e.g., oil pressure, temperature) to estimate viscosity. A faulty sensor or actuator could mislead the ECM into triggering P1521. I recommend checking all related sensor inputs and considering a diagnostic scan with live data to identify which component is sending incorrect signals.

Anonymous 2019-11-23 13:11

Thanks for the detailed breakdown. I’ll go ahead and test the oil pressure and temperature sensors, as well as perform a live data readout from the ECM to see if any sensor values are out of range before replacing any components.

Anonymous 2019-12-10 12:27

I own a similar 2005 Chrysler 300C. These vehicles are very sensitive to oil type — sometimes just opening the hood and looking at the oil cap can indicate whether it's using the correct viscosity or not.

Anonymous 2019-12-10 14:56

I’ve used 5W-20 consistently for years, but once I switched to a full synthetic blend (from a shop), the P1521 code appeared immediately. After switching back to conventional 5W-20 and changing it the next day, the code disappeared. Since then, I've been using conventional oil — yet the P1521 persists. This suggests either an oil compatibility issue or deeper system failure.

Anonymous 2020-06-10 20:50

I recently bought a 2005 Chrysler 300C with no existing codes. When I did my first oil change using 5W-20 with a Mopar filter, the P1521 code appeared immediately. The oil looked fresh and clean — so it seems like the ECM is flagging this as 'wrong oil' even when specifications are correct.

Anonymous 2020-06-11 08:34

After resolving P1521, I started seeing additional codes: P1411, P1416, P1417, and P1418 — all related to oil pressure and flow. Upon inspection, there was significant sludge buildup leading to poor oil circulation. I replaced the oil pressure sensor and connector, but eventually had to replace the oil pump. This suggests that long-term oil degradation or contamination may be contributing to both P1521 and downstream issues. I recommend a complete engine flush and replacing the oil pickup screen after testing oil pressure.

Anonymous 2020-06-11 09:15

Thanks for sharing your experience — very helpful. I’ll follow up with a full diagnostic and consider an engine flush as a next step if P1521 persists despite proper oil use.