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P0300 and P0171 Codes on 2003 Chevrolet Tahoe 5.3L Flex-Fuel: Diagnosis and Solutions

Model: 2003 chevrolet tahoe Fault Code: P0300 Posted: 2017-01-04 19:22

I'm looking for help diagnosing a persistent P0300 misfire code on my 2003 Chevrolet Tahoe with the 5.3L flex-fuel engine (Z VIN). The vehicle has been throwing a P0300 code, accompanied by a P0171B fuel trim issue. I've already performed several troubleshooting steps: cleaned the MAF sensor using MAF cleaner, replaced all spark plugs and wires with OEM parts, checked the PCV valve function, tested for intake leaks with carburetor cleaner (no stall observed), and removed and tested all injectors on Bank 1—each showed strong 4-stream flow. After clearing the codes, I drove for about an hour with a scanner monitoring live data. Everything appeared normal initially. However, while driving on the highway, the P0300 code returned along with the P0171B fuel trim error. Live data shows: - LTFT (Bank 1) above 25%, STFT (Bank 1) over 50% — indicating a lean condition. - O2 Bank 1 Sensor 1: fluctuates from 0.1 to 0.8 when cold, stabilizes around 0 when warm; Bank 1 Sensor 2 remains steady at 0.8–0.9. - All misfire events are recorded only on Bank 1, with Cylinder 1 showing the most activity, followed by Cylinders 3 and 5. I suspect a failing O2 sensor on Bank 1 Sensor 1, but I'm wondering if the MAF sensor could be causing an imbalance affecting only Bank 1. Could a faulty crankshaft position sensor also contribute? Additionally, has this misfire affected my catalytic converter performance? I've noticed increased fuel consumption. I’m considering replacing expensive components before proceeding further and would appreciate any additional insights or diagnostic tips to help narrow down the issue. If possible, I’d be happy to share more scanner data (e.g., idle vs. 3000 RPM fuel trims) or freeze frame details when codes appear.

Related fault codes

P0300

Comments (9)

Anonymous 2017-01-04 23:41

A faulty MAF or crankshaft position sensor is unlikely to cause P0171. The most probable issue is a failing oxygen sensor. Normal fuel trims are essential for catalytic converter efficiency, though no action is needed until a code appears. Could you share your fuel trim readings at idle and at 3000 RPM? Also, please provide Bank 2 fuel trim values.

Anonymous 2017-01-05 11:02

Everything seems to be running fine—started at 0°F, warmed up to operating temperature, and the vehicle runs smoothly. Here are my scanner readings: Idle — LTFT Bank 1: -5.5%, Bank 2: -3.9%; STFT Bank 1: -2.3%, Bank 2: -0.8%. At 3000 RPM — LTFT Bank 1 and 2: -3.1% and -2.3%; STFT Bank 1 jumps from 0 to -0.5, while Bank 2 remains at -2.3%. I regret that I can't attach a picture of the scanner data. Also, the catalytic converter temperature was 100°F higher on the rear side than the front—indicating it's functioning properly (front: low 500s°F, rear: high 300s°F). As noted in my original post, before the issue began, Bank 1 had a long fuel trim of 25% and short trim at 50%, with no reading from O2 Sensor Bank 1. I'm planning to remove the O2 sensor on Bank 1 to test back pressure and proceed from there. This is frustrating but I’m making progress.

Anonymous 2017-01-05 11:04

All oxygen sensors are showing normal readings: Sensor 1 on both banks fluctuates between 0.1 and 0.8, while Sensor 2 remains stable at approximately 0.7.

Anonymous 2017-01-05 11:38

At idle and 3000 RPM, I measure zero PSI before the O2 sensor on Bank 1 (front of catalytic converter). This reading seems abnormal and warrants further investigation.

Anonymous 2017-01-05 13:09

Fuel trims appear normal at idle and 3000 RPM. A possible issue could be an intermittent failure in the O2 sensor circuit or its connectors. Consider replacing the O2 sensors on Bank 1. Back pressure readings between zero and one PSI are within normal range—what seems odd is the lack of measurable pressure at high RPMs. Compare readings with Bank 2 for consistency. If possible, share freeze frame data when P0171 occurs and when misfire codes appear. Also, note which code appears first in sequence.

Anonymous 2017-01-05 13:18

I plan to replace both front and rear O2 sensors on Bank 1. If the issue returns, I’ll post updates. Thank you so much for your help, CJ1—I truly appreciate it.

Anonymous 2017-01-05 13:22

I expected to see a back pressure reading of at least 1–3 psi before the catalytic converter at high RPMs. The current zero PSI is unusual and may point to an underlying issue—thank you for pointing this out, CJ1.

Anonymous 2017-02-18 17:55

I own a 2001 Chevrolet 4.3L Blazer that ran fine until the next day when it wouldn’t start. Both O2 sensors were replaced last fall. I had a bad cap and rotor, which was repaired with new plugs, wires, cap, and rotor. It started right up—but now I’m seeing a P0300 code. This suggests similar misfire patterns may occur in older vehicles with aging components.

Anonymous 2017-02-18 18:50

My 2004 Blazer only shows a P0300 code—no other issues. Double-check your work: did you disconnect any vacuum lines? Are the spark plugs AC Delco brand? Some brands are known to cause misfires.